Will upgrading from 4GB to 8GB make a noticable difference?

Will buying another 4GB of the same ram make a noticeable difference? and if so would I need to change my OC setting/timing/volts etc?

My Setup

Asus P5N-D (Nforce 750i) FSB 1600(400*9) (@3.6GB)
Intel Core 2 Quad Pro Q6600 95W Energy Efficient 2.4GHz SLACR G0 Stepping
OCZ 4GB Kit (2x2048) NVIDIA SLI - Ready Edition DDR2 PC2 6400 800MHz Dual Channel EPP 5-4-4-15
OCZ 600GXSSLI-UK nVidia SliReady600W Power Whisper Technology Active PFC
ASUS Silent Square EVO Quiet CPU Cooler
2 x(SLI)512MB XFX 9600 GT, PCI-E 2.0 (x16), Mem 1800 MHz, GDDR3, GPU 700 MHz(OC'ed alittle)

Thanks in advance!

Since you have two video cards I'm assuming your question is about differences in games. No, you won't see much difference between 4 and 8 GB in games. Read this:
http://www.bit-tech.net/hardware/2 [...] y-better/5

Edit: and yes, if you do add more RAM you may need to change the timings and/or voltages.

You might see a small increase in some games but not much. But I'd imagine in programs such as Photoshop you would probably see some increase for sure. Does this justify jumping to 8gb? That's up to you I guess. Personally, I had the money to blow so I did, I don't know if it makes my machine any better, but it certainly hasn't made it any worse.