SPCR

I'm Going to buy a new computer which psu would you reccomend.

Thermaltake Xaser Silent Purepower 560W PSU Retail Boxed £66
Vantec Stealth Aluminium PSU 520W£62
Aerocool Aeropower II+ Acrylic-Titanium Plated 550W inc.power cord £70
OCZ PowerStream™ 420W ATX Power Supply£70
Enermax EG485AX-VHB(W)SFMA 480W ATX Retail Boxed (Version 2) £70
Enermax EG495AX-VE(W)SFMA 485W ATX Retail Boxed (Version 2.0) £70
Tagan TG480-U01 480W ATX Silent PSU Retail Boxed £64
Enermax EG435AX-VHB(G)SFMA 430W ATX Retail Boxed £64
Tagan TG420-U02 420W ATX I-XEYE PSU Retail Boxed £63
Akasa Ultra Quiet 460W Paxpower ATX Version 2.0 PSU (AK-P460FG-BKUKV2) ------- inc. FREE Aerocool Blue NEON Acryshield £60
Thermaltake Xaser Silent Purepower 480W PSU Black £50
Vantec Stealth Aluminium PSU 420W £48
Thermaltake Purepower BUTTERFLY 480W ATX PSU (Black) £52
Thermaltake Xaser Silent Purepower 480W PSU Silver £52
Globalwin SAF Onyx Silent 18dBA 450W ATX Power Supply £58
Coolermaster Real Power 450W ATX PSU Retail Boxed £58
Aerocool Aeropower II+ Acrylic-Titanium Plated 450W inc. cord £53

I'll be buying a socket 939, sli mainboard
geforce 6600gt ot 6800 maybe 2 later
1 dvd rom
1 dvd writer
4 7200 rpm drives
2 512mb ddr
athon 64 3500

Go to the Power Supply Fundamentals & Recommended Units article and pick a PSU from that list.

Or if you are adventurous buy the Coolermaster Real Power 450W ATX PSU Retail Boxed £58 and mod it with a fixed speed quiet fan.

Only the Enermax is listed (AFAIK). Is there any reason why you can't go to another shop with different PSUs (e.g. KoolnQuiet)? Alternatively find the best PSU in terms of quality/effiency/design and replace the fan - like the Coolermaster that Tibors suggested.

Do NOT buy the 560w Thermaltake. Biggest piece of moderator edit for language ever!

Enermax EG475AX-VE(G)SFMA 470W ATX Retail Boxed (Version 2.0) £60

is probably the best from the selection they have at overclock but I'd suggest you also look at koolnquiet as Chris suggested. (I just ordered a SS-351HT from them)