Sorry for the newbie questions.

I own a Dell 720 XPS running a Core Duo 2.4Ghz ( E6600 ), with 4GB pc5300 677Ghz mem( 4x1 ) with
dual Nvidia 8800 GTX in SLI and a Physics accellerator card. Storage is 2 WD Raptors in a RAID 0.

The motherboard is DELL's version of the 680i SLI with nvidia chip sets,
rated MOBO max is pc6400 (800Ghz) mem and a front side bus of 1066Ghz

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1) I would like to know if changing the memory out to pc6400 800Ghz would make a
noticable differance to performance.

2) how would changing to the 800Ghz mem with a base speed of 400 really affect my computer's speed?
How different would the numbers come out to? Please show me the math.. ( thank you! )

3) Also what if I changed to a Core 2 Quad 2.66Ghz, with the better mem how would that affect me..
please show the numbers again ( thank you!! )

Lastly any thoughts as to an easy swap out upgrade for the motherboard???

I read a different post and the motherboard thing seems to be a no go

Sorry man, I think your stuck with a DELL

By the way, if you were running a 2.4MHz system, it'd better be from the 80s.

1. No, not really. 1:1 ratio (533MHz RAM speed) would be optimal for your system. Try to tighten the timings on your current RAM?
2. 1066/4= 266 FSB. The RAM to match this would be DDR2 (Double Data Rate 2nd generation) 266x2=533MHz. Putting 400MHz RAM in will change the ratio to 266:400, unless you lower the frequency of it (allowing for tighter timings)
3. Again GHz, not MHz . Unless you overclock your CPU, you really won't notice the change. I advise against it. Check your motherboard manual to see how to access OCing options (if they exist). The E6600 can get some nice clocks out of it.

The motherboard swap wouldn't be very painless, so I say it's a no-go. Gotta go now, SATs tomorrow D: