What is the difference between these two chips? And if i'm not in a budget which one should i go for if i'm going to use a 4870x2 and don't plan to run a dual card setup? I'm also using a Qx9650 and going to overclock it.
P45 is fine if you're not going to do crossfire. The difference between the chip sets is that the x48 runs 16x16 slots for full bandwidth crossfire - p45 runs @ 8x8, which is still much better than older x38 boards which run @ 16x4.
All in all, even if you were to do crossfire the P45 scores only ~5% lower than a X48 chip set in most benchmarks.
the p35/45 overclocks the 45nm quad better than the x38/48. i know this from experience with 2 q9450, and 4x x38 and x48 boards, and 3x p35 and 2x p45 boards. and the crossfire difference between an x48 and a p45 is nil. i also know this from experience.
P45 has a 2.0 x16 PCI-E and the second when used is 2.0 x8 PCI-E - if you are just using one GPU (yes, the 4870x2 is one card) then the single slot used will be 2.0 x16 PCI-E. P45 is a better overclocker.
No. A major benefit of a dual-gpu / single slot graphics card is you will get the full performance of both gpus regardless of what motherboard you use.
If your plan is to run a single 4870 X2, then P45 is a better bet. It's a 65nm chipset vs a 90nm chipset, which means it runs cooler and more efficiently. The ASUS P5Q series of boards have proven to be champion OCers, with the top end P5Qs besting ASUS's P45 flagship, the Maximus II Formula. The Biostar I45 board is also very popular with OCers, with people benching FSBs in excess of 600mhz (with dual cores of course.)
It really comes down to "longevity" and used in a CrossFireX configuration. How long do you expect to hold onto your new MB and processor? Is it likely to be more than a single GPU generation?
The reason is, the P45 is the start of the bottle neck (albiet ~5%) in this generations version of the GPUs for multiple GPU card use. Another generation or two and you'll really see the difference between the two chipsets. Some will come back and say if performance is all that important you'll be upgrading to Core i7 (Nehalem) anyways.
However in terms of Core i7, Anandtech and others have shown that the smaller cache sizes does impact game performance, so gamers will be better served at the momement with Core 2 still.
Also FPS improvements in games is significantly more a factor of the GPU than the CPU, at least still in this generation and next generation of CPU's. However, a big determining factor in the GPU relm will be the "mobo chipset" in being able to handle the full bandwidth of that solution unrestricted.
It's possible in two generations of GPU's that low/mid level cards will finally tax an x8/x8 solution versus x16/x16. I suspect based on lower end cards of this gen besting the "ultra" versions of cards two gens back that this trend will continue. Perhaps you'll do a budget CrossFireX setup at that point.
It's worth the small price premium for the X48 over the P45 in my opinion. Several makers also make X48 boards that are DDR2 as well, so you don't have to give up your DDR2 memory if you don't want to. Long term, I suspect you'll find that the additional overhead DDR3 provides will allow for even more spectacular OC's on that new CPU. Try to get a 45nm processor for even more longevity.