Memory is always a cheap upgrade and provides the most bang for your buck.
Your PC can run 4 x 2 GB of PC2-6400 memory for a total of 8 GB, with a 64 bit operating system.
If you plan on installing a 32 bit operating system, then the maximum memory windows can use will be 4 GB.
I would still consider buying 2 x 2 GB of memory for 4 GB so that if you decide to install a 64 bit operating system, you would only need another 2 x 2 GB.
It is fine to have memory installed in excess of 4 GB. No harm will come to the PC--just that the 32 bit OS will only use up to 4 GB of it.
I installed W7 Pro x64 on my dc7800 CMT and maxed out the memory to the 8 GB limit.
The PC is very responsive for hardware that is 5 years old or so.