Ok, you will have to upgrade your Power supply as well. if you want to get a card that is worth anything. In order for us to make suggestions. It would be nice to know what kind of games/activites you are doing or plan to do with this computer. also still knowing roughly how much you have to spend will help to.
this is your memory upgrade information
Memory upgrade information
Dual channel memory architecture
Two DDR2 DIMM (240-pin) sockets
Supported DIMM types:
PC2-5300 (667 MHz)
PC2-6400 (800 MHz)
Non-ECC memory only, unbuffered
Supports 2GB DDR2 DIMMs
Supports up to 4 GB* on 32 bit PCs
*32-bit operating systems cannot address a full 4.0 GB of memory.