Well the X2 def. It's a dual cored chip, so it handle more stuff down the road. It's has a bigger intergrated memory controller built in. (It's got L1 Cache 128KB+128KB
L2 Cache 2x512kb compared to the 4000+'s L1 Cache 64KB+64KB
L2 Cache 1MB). Better for all around performance, but it doesn't overclock that great from what I'm hearing.