check this link
Abit NF-M2S, so if I am not wrong you can put upto 4 GB. i.e. 2 2GB sticks.
You can get single chip of 800FSB 2GB RAM in 1.1K something from Transcend/Kingston.
Buy two chips and remove your 512 MB
Why do I say so, because your RAM is 533 FSB RAM(I had bought 1GB stick of 533 FSB in 1.2K last year), which are costlier than 667 and 800 FSB RAMs, and your CPU is also supports 800 FSB max,