I would agree about the mobo. You need the right Southbridge to unlock that phenom to a quad core. It might be the better deal paired with the 750 chipset that lets you unlock (at least i think its 750... maybe the 710 works but I've never had an AM3 but I did look into that same chip recently). not sure if the 3 core unlocks, but I know that a lot of the phenoms can. Just google the 550 core unlocking to get an idea of the board you would need. I think the problem may be they aren't easy to find any more and they need a specific bios to work. May be more money than its worth for a used board that can do it. As far as cache goes, the APU derived chips are so much more efficient with the level 2 cache they have, that there really isn't much difference in gaming. (assuming the total amount of lvl2 and 3 cache is about the same per core). More cores is better though if you are willing to spend a bit more to get them, though unlocking is never guaranteed. Sometimes you can unlock but the cores are weak and can only do stock clocks or take extra voltage to be stable.
Obviously the trade off here is getting a faster CPU with less cores and a newer mobo, or more cores with a lower clock and older mobo. more cores have benefits for sure, but it is not for sure you would get a quad out of it. IF you ignore unlocking then i still think the 370k is the best bet at the moment. That could change with kaveri. If it makes it out before christmas for you,i would definately look into those. Hopefully this helped some... I know it can be hard trying to fit decent performance into a really tight budget. Hence the fact that none of my mobo+cpu combos I own are over 125 bucks. I buy used and buy slightly older stuff (all FM1 mostly) I wish there were reviews of the 370k myself. Its really an intriguing CPU for the price EVEN on sale for $50 at newegg at the moment
Have to say though that the 750k and 760k are better by a fair margin if you can squeeze in the extra cost.