well the RAM is probably the best quickest/cheapest thing you can upgrade that wont break your bank account and should give a solid boost to overall system performance.
since your using WindowsXP ... if you go from 512MB to 1GB of ram, that alone should boost your overall system performance quite noticeably although getting 2GB (or more) would not be a bad idea
as for the type of ram your using you would need to look into that but it's most likely some form of DDR memory. probably PC3200 (i.e. DDR 400mhz) or slower.
p.s. i have the exact same CPU as you do except mines overclocked 209mhz to a total speed of 2409mhz instead of the defualt 2.2ghz clock speed... not that this matters but i just thought i would mention it