Ensure RAM runs as 2133 and on dual channel. That won't make the CPU faster per se, but it will get the most from it. Going to 8GB of RAM from a lower amount is probably a good idea too, if possible (i.e. your mobo will run happy with it). Keep temps low.
Ensure CPU caches are enabled in BIOS (256K L1 and 4MB L2).
Disable non-essential processes and services to minimise CPU usage or blips. Setup sensible page filing to reduce system resizing page file and constantly having to work at it. Defrag if HDD. Replace HDD boot drive with SSD. Disable indexing. Ensure power profile settings are set to max. Enable any drive write caching, if applicable. Disable automatic updates or frequently recurring scheduled tasks. Disable intrusive or non-critical security (e.g. no need to constantly deep-scan)
If you're in the habit of keeping a browser open while gaming, disable browser add-ons that constantly update you with changes. Play with the hardware browser acceleration settings to see if that helps. Get into the habit of not having a browser open while gaming.
Lastly, some more extreme options:
Perform a clean installation, then do all the tweaking you can.
An 845 will potentially have a 5% improvement over the 880K, and it's not pricey (not sure if a chance at 5% is worth it though).