right now, i'd say the 3700 would play games better than the dual core 3800.
it's straight up faster in speed. most games right now do not utilize the dual cores, so you're not getting much from the 2nd core.
however, games like UT2007 will be creatd with teh dual core processors in mind. so....in the future, dual core will probably perform better than the single cores....i guess it depends on how long you want to keep your setup. probably be a good 2 years before more than a couple games are designed with dual core in mind. if you dont mind getting a new processor in a year or 2, i'd go with the 3700, and get a MB that supports both dual and single...just my thoughts
AMD 3700+ $233
AMD 3800+ $282