DDR2 400MHz vs DDR2 800MHz vs DDR3 1600MHz?

Hello again. I've been wondering lately, is there really an increase in speed (for gaming) with DDR3 memory as opposed to DDR2?

My current RAM is DDR2 @ 400MHz. I suppose I could upgrade that to 800, but would it be worth it? And my motherboard does not support DDR3, so is it really worth it to buy a new mobo and upgrade to DDR3?

I mostly play games, and I don't care about memory elsewhere.
So would there be any noticeable FPS increase if I were to upgrade to DDR3?

I mostly play games like…Battlefield 2, BF: Bad Company 2, Crysis, Far Cry 2, etc…
Basically, do RAM speeds do anything for gaming?

Right now I have 4GBs of DDR2 RAM @ 400MHz. I am upgrading that to 6GBs later this month.

My specs:

Intel Core 2 Quad Q9550 @ 2.83GHz
ATI Radeon 4850 (Also upgrading this to a 6850 later this month)
4GBs DDR2 400MHz
PSU 800W
Windows Vista

etc…

Anyway, so assume I had an ATI Radeon 6850 and 6GBs of DDR2 RAM and the CPU above. Would upgrading that RAM to DDR3 have any impact on gaming? Or is DDR2 fine. As much as I know about hardware, I never have looked into memory…
Thanks, and sorry if I made this post unnecessarily long.

There aren't many 'good' comparisons of DDR2 vs DDR3 -> http://www.anandtech.com/show/2232/7 the differences are primary due to both your GPU and CPU. If you use a high-end GPU on LGA 775 the 'wall' aka bottleneck is primarily the CPU.

You'd be wasting money on your RAM, and only a FEW MOBO's have both DDR2 & DDR3 DIMM slots - so there's probably no way to use DDR3. If you upgrade the MOBO and RAM then you might as well start from scratch and build an LGA 1155 system.

^+1 maybe.

If you're looking at CPU-z http://www.cpuid.com/softwares/cpu-z.html Memory tab and are seeing DRAM Frequency = 400 MHz then indeed it's actually running at 800 MHz.

Example DRAM Frequency = 533 MHz I/O bus clock (MHz) x2 = 1066 Data rate (MT/s).

^Google PC2-3200 ; I'm not the Psychic Network.

Performance would improve by 2-4% at most.

Look at my first post "the differences are primary due to both your GPU and CPU."