My Athlon 64 X2 4800+ system did everything I wanted when I went to Core2, and I loved the system board I had (MSI K8N Neo 4 Platinum), which made it a hard call.
The reason I really did wasn't the CPU change --it was that I wanted to take advantage of the rock-bottom DDR2 prices, and getting two 2GB kits of Crucial Ballistix DDR2800 for $70 a kit (after MIR) was too good to pass up. At the same time, I did it while I'd lose as little as possible from the sale of 2GB of high-end PC-4000 DDR from my old system. If I'd waited a little, I'd have saved some more money on my Core2 processor, but the savings on RAM really did it for me.
If you're running a DDR system, DDR2 is already rising again --best to get a good deal on DDR2 RAM while you can (DDR3 is insanely priced, and probably won't be worth it for another year). Consider at least buying the RAM now, and then holding onto it until the rest of your components shake out. As for Penryn, early samples look promising, but across the board it's still only a 5-10% increase on average. Wait if it will save you money, or if your existing system can do everything you need. Buy now if your system is struggling to perform the tasks you want/need it to, and don't worry that you didn't get Penryn.
Core i7-4790K, GIGABYTE Z97X-UD5H-BK, 16GB (4x4) G.Skill RipJaws PC1866
Corsair 650D case, Seasonic X750 Gold PSU
WD `Raptor 600GB boot, WD Caviar Black 1TB data, NEC 7200 DVDRW
2x XFX R9 280X Black in Crossfire, X-Fi Titanium, Dell 2407WFP