My Macbook begin to RSS, and I really need a stable mac, local microcenter has core solo mac mini for $549. so my question is: how much performance gain from core solo to core duo for normal work, like internet, java based program, MS office, etc.
A Mac Mini with the 1.5GHz Core Solo would be okay for your needs, but if you can, go with a Mac Mini with a Core Duo processor. The marginal difference in price is easily outweighed by the added performance from having a dual-core processor.
What about an iMac? You can get an iMac 17" Combo Drive for USD 925 at Amazon (with discount), or the SuperDrive version for USD 1125. So don't have to worry about the screen, and also get iSight.
out of interest, have you called Apple about your RSS? They can fix you up with a new heatsink/logic board for free. Then you won't have to spend money getting a Mac Mini, display, keyboard and mouse.