that socket is a socket 939. there isn't an awful lot that's faster you can put into a socket 939. everything else is moving to am2 or another socket form factor.
in terms of processors that are socket 939, there are maybe 2 amd athlon x2 cores, and a couple of other slightly faster and more expensive cores available for 939. the fastest out there for 939 is looking like the opteron 185 denmark 2.6ghz, with a 1mb cache. it's $230. ouch. apart from that, there's the amd athlon 64 x2 3600, 3800, and 4200. the dual cores would certainly be faster than the san diego core you have. but other than that, your choices are limited with a 939. if i were you, i might consider buying both a new processor and a new motherboard so you're prepared for the future.
a motherboard with an AM2 socket will allow many more upgrades in terms of processors, and you might have to replace fewer parts in the long run.
the amd athlon 64 x2 5200 is a very nice processor, about 120 bucks or so on newegg.com. and an ASRock aliven nf6g vsta motherboard for an extra 54 bucks would put you way up there in terms of power for relatively cheap. after all, since that is your main/gaming computer, i would want to keep that thing as up to date as possible in the first place.
it's up to you though. that's what i would do in your situation. you're replacing the main board right now anyway. why not go all out? unless of course you're tight on cash.
just in case you're interested though, here is the link for the processor:
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103759
<-- processor
and for 20 bucks more, you can upgrade to a 2.8ghz instead
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103771
<-- amd athlon 64 x2 5600 windsor
those require a mobo with an AM2 slot. but i can't recommend one to you without knowing what specific type of RAM you have. pc2 6400, ddr2 800 RAM?
if it is, i think that mobo i mentioned earlier is perfect for you.
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157108
<-- mobo
otherwise, disregard the mobo for now until we can find you one that supports your ram type.