LOL, well the $45 part should clue you to the fact that you got a bargain processor. For many tasks it's perfectly capable, I'm just saying it's not ideal for gaming...especially newer games.
Most "good for gaming" processors are going to run you $100+ right now. I think the C2D 3GHz I just got rid of runs about $170 right now. Since you already bought the mobo, look at what the board is capable of running on Asus' site and you'll see what CPUs it could take.
If you stick with older sims it shouldn't be too bad (you likely can't run at max detail), but I think DCS: Black Shark and Oleg's upcoming SoW:BoB would bring that to its knees rather quickly. The funny part is your GTX280, a card that's faster than my 260, is way fast for a Sempron. I literally spent the same amount for my video card and CPU ($260 each I think) to keep the performance "matched". In your case, your CPU will be the bottleneck, there won't be anything it could throw at a 280 that the 280 can't handle. Of course, when you spend $45 on a CPU and what, $350+ on the video card, that's what will happen.