I'm in a similar situation, and have been keeping a pulse on Ivy Bridge since last May when I started thinking about upgrading (ended up just getting a GTX560Ti to replace my GTX 280 at the time).
I'd say wait for the following reasons:
- Release is rumoured to be week 14 (April 1-8) which is now only 2 months away.
- Z77 motherboards bring guaranteed PCI-e 3.0 (useful down the road if you keep your motherboards for a long time like I do) and integrated USB3. Still only 2 Intel SATA3 ports unfortunately. Some Z77 will have Thunderbolt if that matters to you.
- Ivy Bridge CPUs will have lower power usage (77W TDP vs. 90W on the quads), so less power and less heat
- Better iGPU
- Small IPC improvement
Downsides to waiting:
- Won't be able to get a great deal on CPU + Mobo like you can with the 2600k/2500k + Z68 at Newegg/Microcenter/NCIX
- Possible price premium on motherboards, CPUs will launch at standard pricing like SB did, new chipset MBs seem to have a $25ish price premium for the first few months
- In real world desktop usage/gaming, highly unlikely to be any discernible difference
Since this is a hobby for me (researching, building, testing, overclocking), I don't mind waiting to have the shiniest new parts, since I keep my systems for so long with only GPU upgrades. I've been watching for deals and have started assembling parts already such as PSU, heatsink, case, SSD, RAM.
The i5-3570k is what you will be looking at (2500k replacement SKU):
http://www.anandtech.com/show/5166/ivy-bridge-overview