diablo2 if you want a really challenge in co-op mode ... there is a patch that is gona be released for d2 which might cater for higher res ... i dont think there is another rpg game comes close to the difficulty level in hell of d2, you need co-op to finish hell ...
titan quest is also brilliant for co-op mode but lags after an hour's play, since the people that make the game broke up there was no patch to fix it ... there was however a community patch that fixed alot of issues with TQIT, you can give that a try ... another thing, if you go the TQIT route, use the xmax mod for some hectic fun and challenges
neverwinter nights 1 is also cool, tho its kinda too slow for my liking, but once you get into it, its gona drain your life away ... i dont think neverwinter nights 2 is co-op hey :|
