I'm using Blender 3D's internal render engine- It is CPU only and is highly multi threaded.
All the cpu's that I am considering an upgrade to have both more cores and a higher clock speed, I'm not sure how my gaming performance would go down.. Is it even possible? I know that there will be no significant or noticeable improvement with games if i keep the GPU the same, but the CPU rendering should go up 150% at least right?
X6 chip is same size and 45nm process as x4. So you take same amount of power, same bandwidth, but divide by 6 instead of 4. Per core performance suffers, hence the x6 CPU's are worse at gaming than x4's.
I suggest, if you do upgrade cpus, to wait until Sandy Bridge comes out. Which is next month. Pricing has been confirmed that the i5 2400S will be at 199$ so the old i5 750 pricing. This puts it at a great spot, the reason being the performance of the 2400S is at that of the current i7 880, or for that matter the top of the 1156's lineup asside from the 875K.
Other option might be to pick up a phenom 2 x4 be should be cheaper, but give you some better game performance, plus they've got higher clock speed. It appears though you have around 200 to spend.
Maybe pick up an athlon II x4 640 or similar, that way you go from 2.6 ghz to 3.0 or better. with the rest, maybe grab a gts 450. That would fit the budget, and should give a nice overall upgrade. Or OC the 620 to close to 3 ghz, grab a 460 and call it a day.