*I believe the highest supported CPU is the:
i5-2500
It's approximately $100 used on EBay. Unless you have a dual-core or significantly weaker quad-core there's not much point in upgrading. If gaming the i5-2500 isn't too bad.
An i5-4590 is only around 15% faster and it's rarely a game bottleneck.
If you want better advice:
- current CPU?
- reason for upgrading? (gaming, video edit..)
- if gaming, what GPU do you have (if crappy, it may make more sense to buy a GTX750Ti for example)
The stock cooler may also get pretty loud when the i5-2500 heats up. There are nice, small coolers but it's hard to recommend spending too much money without knowing details.