I had this same problem with Warcraft III a long time ago. RAM was the first thing that I went after (After updating drivers and such). After removing one of the 512s, it was fine. No problems since. Everytime I get a new update from WarCraft, I'll shut it down, add the additional 512 and see if it was somehow "miraculously" fixed in the software..... but I'd wait about 1 or 2 games and then I'd get kicked out with a nasty error. Nasty, natsy error! Like one of those where a developer would say "Man......WTF did you do???" LOL
But, it fixed it everytime by removing the extra RAM.