As stated by BFG, for old socket 775 cpus the memory controller was still on the motherboard. You will need to lookup max specs on the board. My educated guess is that 2x2gb or 4x2gb would work.
That will be using DDR2 which will be difficult and expensive to find quantities of even 4gb.
You have to be very careful with ddr2 memory. A lot of the actions on ebay for ddr2 memory are for "high density" memory and these will not work on intel systems like yours. If it is not from a name brand (crucial, kingston, hynix, g.skill, adata) then it is likely that high density crap.