If you are wanting SLI with a P55 board, then you really want a board that does x8x8 PCIe and supports SLI.
This board is probably the cheapest that has all these features, it also overclocks well.
However, I should draw your attention to a recent drop in the price of the
retail i7 920 CPU. At the moment this chip is £16 more expensive than the i5 linked above (and £15 more than the retail i5) - the i7 isn;t any faster in games (at the moment), but it does have hyperthreading enabled so it is faster in multithreaded tasks and overclocks easier and with less volts.
Here is a comparison of a stock i5 750 and i7 920 in a variety of tasks.
A good X58 motherboard is also slightly more expensive than the P55 I linked to above,
this one is £32 more. However, for this money you do get more features - it runs dual graphics cards at
x16x16 PCIe, supports Dual channel and
Triple channel memory and overclocks easier than the P55. As well as this, the X58 supports 32nm Hex core i7 chips if you want to upgrade the CPU further down the line and the extra PCIe bandwidth means it will work with contemporary high-end graphics cards for longer than a P55.
All-in the i7/X58 option is £48 more expensive than the P55/i5 route (+ £20 if you want to go for 6GB triple channel RAM instead of 4GB dual channel). It is up to you whether you think this extra expense is worth it.
