I found this link last night
http://www.notebookcheck.net/AMD-Radeon-HD-6630M.43963.0.html if you got the version with the radeon 6630m, it looks like it does pretty well gaming wise!

(check out about 1/2 way down the page for various games).
As for a gaming mouse, it really depends how much you want to spend.
Beings you bought a mac mini, I guess you arnt too hard up lol. So if you want a kickass mouse, the Razor Mamba is great!. Heres a random review
http://reviews.cnet.co.uk/mice/razer-mamba-review-49303086/
I'v had 1 for a year or 2, and the only thing I can find even slightly wrong with it is it's a bit fiddley to plug and unplug the cable if your mouse runs out while playing. Also you get soo used to using it wireless, that it feels annoying having to use a wired mouse again lol. Saying that, unless you spend all day playing world of Warcraft (like I used to...), you probably won't run out of battery while gaming. Just stick it on the charge cradle thing when not using it.
Saying that, I ain't 100% sure the mouse works on a mac mini, I'v yet to get 1 lol. As far as I know, you can use any mouse on a mac mini. (hopefully I'll be buying a mini soon!)
As for the "wired is better for gaming" these days there ain't much difference unless you are a pro fps player who goes to competitions lol. The mamba is apparently polled at 1ms! (sais here)
http://store.razerzone.com/store/razerusa/en_US/pd/productID.169418100
*edit* if you get a cheap wireless mouse, then it probably will have alot slower response time lol.
Oh yeah, I watched a Starcraft 2 video on a mac mini on YouTube, it seemed to look great!, I didn't notice any slowdown. (I'll find a link to the vid if you can't find it)
*edit*
here's the youtube vid of Starcraft 2. It's using the mac mini with the i5 and radeon 6630m
http://www.youtube.com/watch?v=mhTwoDNL3Ps I guess it'd run even better with an i7!