My CPU is Core2Duo E7400 and I thought about doing a whole new build but was wondering if upgrading just the GPU from GTS 250 (512mb ram version) to GTX 560 and also getting a SSD for a boot drive would yield a nice performance boost to last another year or so, or is the CPU just too slow that it'll be a bottleneck? I'm not too concerned about playing the latest games, it's just that when I'm rendering a video I really can't do anything else on the desktop because the CPU is lacking.
A GTX 560 ( not a Ti ) is just a rebadged GTX 460 1GB that is highly factory overclocked. I orignally bought my GTX 460 1GB when I was running a Core2Duo E8400 and there was no bottlenecking. I overclock my GTX 460 to GTX 560 levels and have since day one. I did have that E8400 overclocked to 4Ghz though. I also had that system when I bought my SSD almost 2 years ago. It made a huge difference.
Now all that being said, if rendering is your main concern none of that will help. You need raw CPU power for that and the only way to increase that is a full upgrade. Your best bet for rendering is an i7. Either a 2600K or a 3770K.