since ur building a comp right now, and not deciding whether to upgrade or not, i would buy the x2 3800+. i was in the same predicament ur in right now, but im not much of a gamer so i went with the x2. basically, if you're a hardcore gamer, go 3700+. and when i say hardcore i mean HARDCORE. honestly, .2 or .4 ghz wont make much of a difference in gaming (more dependent on ur gpu i think) and its nothing u cant overclock to overcome.
i would get the x2, solely because gaming will be about the same (maybe 10fps difference tops? - and at 70 fps, ur not gonna notice a difference) but itll give u a lot better multitasking than the 3700+
btw, if u get single core, id get the 3000 and oc it =)