As always, it depends on the use. For most uses, there might not be much difference at all. Faster harddrives only pay off when your doing tasks that access the harddrive. (as much as I try to tell people, GAMES don't qualify.) If you setup an AID0 array on the 2 250GB drives, you might get the game levels to load 2 or 3 seconds faster. (depending on the game, it might be less then one second faster...)
I'd keep it simple and just get the 500GB drive. No RAID drivers to get corrupted, and its just the single drive. (less heat, noise, electricity, etc) RAID is for heavy use servers, not single use desktops.