I'm guessing it's mid range gaming (CSS, WoW, etc)...so here's a few.
AMD (socket AM2, AM2+, AM3)
Intel (Socket 1156)
Intel (Socket 1366) Top dog, i7 processors...probably overkill
The AMD is the cheapest mobo. AMD everything is a lot less expensive but always a good choice for mid range gaming. If you're talking about Crysis and Stalker and you have the budget, bump it up to the 1156 and get an i5 processor. And if you want play those games and create Jurassic Park 4 (heavy video rendering), go with the 1366 socket and the i7 920. Also remember, the intel motherboards don't have onboard video graphics. If you're gaming, you need an aftermarket video card anyway, but with those mobos, you'll need to make sure you have them when you build it. Otherwise, you can't plug in your monitor.
These are just suggestions. I'd go with the AMD unless you're doing really intensive gaming. Performs great for the cost. Oh and I just got to that...if you want to straight up just upgrade the mobo...like Foothead said, we'll need to know what socket or CPU you have in now.