Win XP 64 bit for gaming was not the best solution drivers were always a nightmare
As for your ram adding in more ram of different sizes and brands is just asking for issues - and highly likely to slow your system down
Dump the 512 if you plan this option
Ram comes in 512, 1024, 2048, 4096, 8192 sizing there are no 3G chips to mu knowledge
Make sure he gets the right ram for his MB check which type is required and voltage.
Remember to use the slowest Ram in the First slot - faster ram will down clock slower ram becomes unstable when overclocked.
Sry mis-read 512 GPU - better GFX card always helps for games more so than ram
As for 512/1G/2G on GFX it not critical the Ram speed is more important and higher the Resolution you play at the More GFX Ram is needed for textures
Edit... Win Xp it stuck on DX9 any GFX upgrade would need to cater for this...