Is it worth upgrading to a Dimension 5000 (taking the existing HD and RAM from the 280 into the 5000), if that's all I can afford? Is there an official benchmark bar chart between the two?

Paulji786,
Below are the specs you requested on these 2 systems.
Optiplex GX280
Memory
Type
400 & 533 MHz DDR2 SDRAM
400 & 533 MHzMemory connectors
small form-factor computer: 2small desktop computer: 4desktop computer: 4small mini-tower computer: 4mini-tower computer: 4
Memory capacities
128 MB, 256 MB, 512 MB, or 1 GB non-ECC
Minimum memory
dual-channel: 256 MBsingle-channel: 128 MB
Maximum memory
small form-factor computer: 2 GBsmall desktop computer: 4 GBdesktop computer: 4 GBsmall mini-tower computer: 4 GBmini-tower computer: 4 GB
BIOS address
F0000h
Dimension 5000
Type 400- and 533-MHz DDR2 unbuffered SDRAM
Memory connectors four
Memory capacities 256 MB, 512 MB, or 1 GB non-ECC
Minimum memory 256 MB
Maximum memory 4 GB
System clock 800-MHz data rate
Memory speed

Thank You,
Dell-Jesse L
Dell Social Media and Communities

2 gygabytoes are usually the max, you could upgrade the OS to maybe Windows XP 64 bit, but this isn't worth it in such an olde machine! Throw it out and get yourself an ASUS laptop with Windows 7 64-bit, a solid state HD and 4 or more gigabytes of ram instead!.
http://www.crucial.com/upgrade/Dell-memory/Dimension+%2F+Dimension+XPS/Dimension+8200+Series+%28400MHz+FSB%29-upgrades.html

Upgrading HDD is no risk for your PC. The RAM memory instead should cause a problem if it's not compatible. The problem that can appear is simple: your computer won't start until you take out the ram memory. Your type of motherboard supports:Max 2Gb of RAM (DDR SDRAM)
Cheers.

You can install a GIANT drive, as you have a suitable processor to handle 1TB if you want.... so a 250GB, 500GB drive will work like a charm, also if you upgrade the Hard Drive, you should also put as much RAM as the machine can take, and you can afford, 4GB would be real good, and speed the machine up more than any other single thing, also get a high speed, 7200RPM, BIG, 8-16MB, cache drive too..

Your PC can support upto 4GB of RAM incase you are using vista or Windows 7 and if you are using win XP (32 bit) then only 3 GB is supported 1GB shall standby and also make sure that they are not more than 533 HHz
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Thanks And Regards

Well it seems you can go up to at least a 3.6GHZ processor. It is socket 775, so any Processor that will fit will in all likelihood work fine. As for the hard drive well I like 7200RPM 8MB Cache 3GB/sec SATA drive. or similar spec, & truthfully one brand is as good as another... But if forced to say.. well Seagate.
Also if you want a fast machine RAM as MUCH as you can put onboard and afford.
I have found a manual here for you. http://support.dell.com/support/edocs/systems/dim9100/en/D86630LRs.pdf

I hate to be the bearer of bad news, so don't 'Shoot the Messenger'!

The Dell Dimension 8200 Desktop PC uses RDRAM ram memory.
VERY expensive stuff!

Supporting Data:
This is Dell Support, Manuals, and the Dimension 8200 Service Manual,

http://support.dell.com/support/edocs/systems/dim8200/

Scroll down the list to the heading - Specifications, and left-click on it.

Scroll the page down until you come to the heading - Memory
Now look at the first subheading - Architecture
RDRAM

You have 4 ram memory slots. (memory connectors)

EACH, ram memory slot can support up to a Maximum amount, of 512MB. (512 MegaBytes)

TOTAL Maximum Amount for your computer is 2GB. (Not shouting)

1024MB = 1GB. (1024 MegaBytes = 1 GigaByte)

The type of Rdram ram memory is PC800, and it is Non-ECC.

(Error Correcting Control, or also stated as Error Correcting Code.
Used mostly for Server computers. Your computer does not use ECC ram memory)

In My Humble Opinion, (And again, don't shoot the messenger), it would be better money spent to buy a decent affordable computer, than it would be to upgrade the ram memory in this computer.

This statement is based upon the fact that the Dell Dimension 8200 ships with 128MB's of ram memory.
(Makes it a pretty slow computer. Rdram or not)

The average cost to install, for example, 1GB of ram memory is OUTLANDISH! (Not shouting)

Example 1:

http://www.4allmemory.com/index.cfm?fuseaction=search.memorySearch&model_id=24927

Example 2:

http://www.pcmemorystore.com/Dell-Dimension-8200-ram.htm

For about another $100 you can buy a pretty decent new desktop computer, that will run the one you have in circles.

Just my thoughts.

Your existing CPU will not work with that motherboard. The AM2+/AM2 processor slot is for AMD processors only. For this motherboard, you will need one of the following processors: Phenom FX / Phenom / Athlon 64 FX / Athlon 64 X2

HI,
You can go into the BIOS , under onboard devices. Change the primary video controller to PCI.
refer to the following screen
http://supportapj.dell.com/support/edocs/systems/dim4700/sm/syssetup.htm#wp1052347

If the cards are not detected on the board. Then the problem could be of the PCIe slot .


Thanks
Proton

First step : Make sure the memory you purchased meets the following specifications

"168pin PC133 SDRAM DIMM" which should be easily accessible from wherever you bought it.

If not, then you can find memory here. that the company guarentee's will work with your system. Also - You want to make sure the memory that was in your system to begin with is identical to the memory you are sticking in it to get it to 512MB Systems that are this old with "PC133 SDRAM" are very picky about what kind of ram you stick in them. Make sure wherever you purchase your memory from, has a guarentee that it will work in the make/model of your computer.