Building my first PC and I have a pretty good build in mind for under $550. The current CPU is the AMD Athlon II X3 455 Rana which seems pretty good for a budget build, and gives me some money left over for a 6850, but for an extra $30 is the FX-4100 worth it? I've heard mixed reviews of the new Bulldozers so I'm a bit unsure about it.
If you just want to game i'd go for the cheaper Athlon II X3, as the only benefit i could imagine that the FX 4100 would give you would be in software that takes advantage of it's available resources such as the extra core it has.
You could also get an Athlon II X4 640 for $100, you could always skimp a little on the motherboard unless you need the features the more expensive board gives you.