Vista - 5/5
Intel Core 2 Quad Q9300 2.5GHz - 4/5 (Q9450 for the 5):http://www.aria.co.uk/Products/Components/CPUs+%2F+Processors/Intel/Core+2+Duo/Intel+Core+2+Quad+Q9450+2.66GHz+(OEM+775)+?productId=30934
750W PSU - 4/5 (800W+ for the 5):http://www.aria.co.uk/Products/Components/Power+Supplies/500w+%2B/Xilence+800w+Gaming+Edition+PSU+?productId=26716
4GB RAM - depending on speed and setup between 3/5 and 4.5/5 (4GB dual channel DDR2-800 or better):http://www.aria.co.uk/Products/Components/Memory/DDR2/DDR2+800+(PC6400)/Patriot+4GB+PC2-6400+C4+Extreme+Performance+(2x2GB)+?productId=29114
1TB HDD - 4.5/5 (F1 1TB for the 5):http://www.aria.co.uk/SuperSpecials/Other+products/1TB+Samsung+Spinpoint+F1+SATA2+7200RPM+32MB+cache+?productId=30998
2x 8800GT in SLI - 2/5 (get a ATI HD4870X2 in single GPU for the 5 - multi-separate-GPU is very, very bad at the moment): http://www.aria.co.uk/Products/Components/Graphics+Cards/PCI-E/ATI+Radeon+4000+Series/Sapphire+HD+4870+X2+2GB+PCI-E2.0+?productId=33273 (if this is too expensive then settle for a normal 4870).
Due to your SLI setup you may only be able to run Crysis on High with 2x AA, but with the setup I've given you using the 4870X2 and some serious cooling and overclocking, you should be able to run Crysis at Very High detail and 8x AA.