You have DDR 2 RAM now. It may be that your motherboard will accept ONLY DDR 2 RAM.
DDR 2 is more expensive than the newer DDR 3.
You can get 4 GB of DDR 2 in the USA for roughly $50. That's enough for most users.
The second thing I'd recommend is an SSD--solid state drive, for maybe $100 US.
That's an old PC. I would be careful about putting money into it, rather than rebuilding it with a new motherboard, new CPU, and the newer DDR 3 RAM.
You can get a pretty powerful upgrade of those 3 primary parts for $400 to $500 in the US. Might be higher in your locale.