OK, so you have some Sandy Bridge (2nd-gen) & Ivy Bridge (3rd-gen) options available (see
here). Personally, 3rd-gen would be the better choice: newer, & more likely to be in stock somewhere.
However, you're already at 2nd-tier on the
CPU hierarchy, so it's going to be tough to get a good upgrade. Best suggestions I would recommend:
- Core i5-3550: 10% improvement in base core speeds (3.0 to 3.3GHz), 13% improvement in 4-core Turbo speeds (3.1 to 3.5Ghz), & a 19% drop in TDP (95W to 7&W), & it should just drop in (same BIOS version)
- Core i5-3570: 13% improvement in base core speeds (3.0 to 3.4GHz), 16% improvement in 4-core Turbo speeds (3.1 to 3.6 GHz), & a 19% drop in TDP (95W to 77W), & you move up to a 1st-tier CPU. But you'll have to update your BIOS first (needs version 0803).
- Core i7-3770: same per-core improvements for base & Turbo speeds, same drop in TDP, same BIOS version, plus you go from a 4C/4T CPU to a 4C/8T CPU.
- If you can't find anything else, you can always get an i5-3570K or i7-3770K. However, while you get the faster speeds, you're picking a more expensive CPU because it can be overclocked, but your motherboard won't allow overclocking, & you'll have to shell out for a separate CPU cooler ("K" CPUs aren't sold with their own coolers).