You'll be overclocking? I recommend the
Asus P5K Deluxe (or
Premium, if your a rich bugger) as they are VERY nice overclocking board. If you need SLi then go for
this, but it won't overclock as well.
CPU wise an E6850 can hit up to 4 GHz on air cooling, whereas a Q6600 can "only" hit 3.6 GHz max on air cooling, however as more and more games become multi-threaded i think you'd be stupid - you might "loose" .4 GHz in the short term, but by the time you actually need that extra power 2 extra cores will help more.
Personally i'd get a
Q6600 G0, as the next Quad up, the QX6850 is VERY little increase (when overclocked) for the amount of cash. With Penryn on the horizon you've "saved" something for it, too.
If money is no object then a new PSU is a must - the one i linked too is good, but
this is a better bet (yes i know 800w is overkill, but Tagan pwn, and anything from Seasonic or PCP&C thats 650w+ is the same price).
What cooling will you be using to overclock? If your looking at air cooling then i suggest that you get some good stuff, ie a
Tuniq Tower, a
Thermalright HR-03 and some
Arctic Silver 5. After looking at your case it might also help to retro fit some new fans in - 3x of
these and one of
these - they will not only be quieter, but also move a fair bit more air around your case. If you want to spend a little more for more cooling prowess, a water cooling kit might be a nice idea, the Swiftech Apex Ultra is pretty much the only kit i'd recommend, and will only do the CPU, however i think you can add a GPU block, however at £160 it's a little steep, and it's outta stock everywhere.
I'd personally get this
8800GTX as it's the cheapest.
I'd also get 2 GB of
Crucial Ballistix PC6400 Memory, although it's
PC8500 brother is another option. If your feeling like a posh bastard 4 GB might be a better option, but it's probably best to buy it when you need it.
If your going to spend so much (ie £400 on a CPU) and the best kit to buy is so under your budget, i'd recommend doing stuff like replacing your old Maxtor with this
Seagate as it's faster, and this
sound card rocks, if you don't already have one.
If you change your number of cores or your motherboard you will have to re-install Windows. Perhaps it might be time to consider XP 64 or Vista?
Hope This Helps!