What RAM should I buy?

I want to buy a RAM for my PC. I think 1 GB would be OK(but a 2GB with a reasonable price would be even better). It must be a DDR1 also. Here are my system specs: Windows XP Home sp2, Intel Pentium 4, CPU 3.40 GHz, 512 MB of RAM(for now)...Can you recomend a good one? Thanks for taking a look 
