I7 vs i5 for digital editing

Due to my faulty motherboard I'm thinking of swapping the mobo/CPU combination to something more up-to-date. Currently it's Asus P5QD Turbo with Intel Q6600 and EVGA NVidia 8800GT which started failing although it may be a combination of failing graphics and motherboard. Still running the system using a PCI card but any PCI-E card I tried and I tried 3 so far just won't work properly displaying some artefacts even right after the system startup although one run fine in Safe Mode. Anyway, I'll be using the system mainly for photo editing using Lightroom and Photoshop and maybe some video editing. From what I heard it's not as much the graphics as the CPU that impacts the performance of these applications. Since right after the Christmas is not the best time to for unplanned spendings I don't want to just throw out the money without a good reason. From some comparisons I see the 3370K performs much better than any i5 but I'm not sure how it looks in real life. If it means rendering a scene 1s faster it's not worth spending the extra for me. If it's 10s faster that's a different story. I'd like to reuse as much as I can from the old system so Antec 650W PSU stays and all my drives running at around 150MBps at best would stay as well.