My Radeon card isn't recognizing the monitor and Catalyst won't read its full resolution (1024 x 768).

Hello there
The HD 2400 is a pretty old card and I doubt it will give brilliant graphics.or will it run DX11
However.
1)Have you disabled the 'on-board' graphics in your BIOS settings ? Your computer will not recognise it if the old on board chip is enabled.
2) Have you tried connecting the 2400 using the 15 pin video connector ?
3) You say it isnt 'recognising' the hd2400 and yet you are saying the monitor is working at 640 !!!! Which is it. Does the monitor work on the old onboard output or the HD2400 ?
4) The version of catalyst depends on your operating system and whether 32 or 64 bit.
Use the right one via here http://support.amd.com/en-us/download
and uninstall the old one first..
5) If the monitor does not work at all on the HD2400 try the card in a different PCI slot.
6) ALWAYS check after you start the PC that in 'Device Manager' that the display adapter (hd2400) is recognised.
7) run ' DXDIAG ' to see how good the monitor is. (type in dxdiag the start orb search box)