PC2 Memory question

I have a Gateway GT5636e system... okay, not the latest and greatest, but I'm not looking to do a major upgrade. I upgraded it to Win7 64-bit (had Vista 32-bit previously) and I'm looking at the memory right now. At this time, I have 2Gb of PC2-6400 rated memory and 2Gb of PC2-5300 (obviously, faster memory is running at the lower speed right now). My question regards this... the motherboard will allow up to 4Gb of PC2-6400 memory, or up to 8Gb of PC2-5300 memory. Would I be better of going with the 4Gb of faster memory, or going with the greater size of the slower memory. If I went with the latter, I'd initially go 6Gb, so I guess it would be a 4Gb of 800Mhz versus 6Gb of 667MHz memory.

Other info... my processor is an Intel Quad Q6600. Corsair PSU at 650W.

Thanks!

Unless you are doing something where you would benefit from more that 4gb of memory, like video editing or running virtual machines, I would probably just stick with what you have now.

You will not really notice any difference from a small speed bump of PC2-6400 over the 5300. An SSD for the OS would probably give you the biggest bang for your buck.