What will you do with the new pc?
If you want to play games, spend your money on a good vga card. The 8800GTX, 8800GTS-512, or the 9800GTX are all in about the same performance class. They are about as good as it gets, and will play any game reasonably well.
SLI has been a poor upgrade path in the past. It should be used only by
those who will not currently be satisfied by the fastest available single vga card
which is currently the 9800GX2.
To get SLI. you have to spend more up front for a SLI capable mobo, and a
more powerful SLI capable PSU. Upgrading a single card later with a
second equal card does not get you 2x increase, it is more like 1.1x to 1.5x depending on the game.
At that time, you will still be paying top dollar for a card that is closer to
being obsolete. A higher end card will likely appear before summer. It would be better to sell the old card and use the proceeds
towards a better new generation single card.
The performance advantage of pci-e 2.0 today is miniscule(<1%?). My the time that changes, nehalem will be upon us, and current motherboards will be obsolete.
EVGA,XFX and BFG are considered the best vendors.
No, you can't add the mhz for multi core cpu's.
At the level of the E8500 or Q9450, the vga card is much more important for gaming than the cpu.
At that level, overclocking is good for bragging, but it will not net you as much increase
in FPS as a better vga card will. Today, very few games can make use of more than two cores.
Flight simulator X is an exception. It is not a trivial matter to code multi threaded programs,
and game vendors will not sell too many games that require quads to run.
I don't see this changing in the next couple of years, and then nehalem will be upon us.
Net: E8500 for the increased clock speed.
re: memory for the C2D:
Right now, DDR2 memory is king. It is MUCH less expensive than DDR3.
4gb of DDR3 costs $540.
4gb of DDR2 costs <$100.
The C2D processors are not very sensitive to memory speeds.
Real world application tests(vs. synthetic benchmarks) show
no difference in performance between DDR2 and DDR3 memory. In fact,
there is less than 2% difference between the slowest and the fastest modules.
If you are trying for record overclocks, then all bets are off, and faster is better.
In my opinion, it is unwise to build a system around DDR3 today.
You are better served by starting with 4gb of DDR2-800 memory. Eliminating
just a few hard page faults is worth it.
Pick a 4gb kit in a 2x2gb configuration. It is usually cheaper than a 4x1gb kit, and you preserve the option to go to 8gb.
ASUS and Gigabyte seem to be the favored motherboard vendors.
MY suggestion for gaming:
P35 based motherboard
4gb of ddr2-800 quality memory in a 2x2gb configuration.
E8400 cpu
9800GTX or 8800GTS-512 vga card
A quality 500-600watt psu from corsair, seasonic, or PC P&C. or a tier 1 or 2 unit from this list:
http://www.xtremesystems.org/forums/showthread.php?t=108088
Onboard sound, at least to start.
---good luck---