Hi killiebrad, the reason why I opted for the Intel Core 2 Quad Q6600 over the Intel Core 2 Quad Q8200 mainly because of how easy the Q6600 is able to overclock due to its 9 multiplier compared to that of the 7 multiplier that the Q8200 has. Also, in general, the Q6600 seem to overclock very well. The Q6600 also has double the cache of the Q8200 which is a nice bonus.
Regarding the motherboard – The ASUS P5Q Pro really is a fantastic board, very reliable, a great set of features and very easy to use BIOS. The ASUS P5Q Deluxe in my opinion isn't worth the extra £40 or so since the ASUS P5Q Pro should be able to offer everything you will need. Unless of course the Deluxe has a certain feature that you would consider is worth paying the extra £40 for. The main power connector is also in the right place so you shouldn't have any problems there.
The ATI Radeon 4830 isn't such a great card for gaming and you would be far better of spending the extra £15 on the ATI Radeon 4850 which is a very good graphics card for the price.
If you are looking at spending in the region of £1000 then it would be highly advisable to get 4GB of memory instead of 2GB. The only real reason to opt for PC2-8500 rated memory is if you have a CPU that has a low multiplier which the Intel Core 2 Quad Q6600 doesn't or you are looking at doing some extreme overclocking.
Though, if you are looking at overclocking the Q6600 past the 3.6GHz mark then it may actually be worth getting some PC2-8500 memory which will actually take the Q6600 right the way too 4.5GHz without the need to overclock the memory. However, once you get around the 3.6GHz mark, providing you have a chip that is able to clock further, you then have to take into account temperatures which will untimely be the limiting factor.
I'm sure if you were to purchase PC2-6400 memory though, it would be able to overclock past its rated speed. However, this isn't guaranteed.
Now, the reason why I have changed the CPU cooler is because the Thermalright TRUE Black 120 is only actually around £10 more, once you have included in the fan, but is one of the best air coolers you can get. It is highly regarded.
The Corsair HX 520W ATX2.2 Modular SLI Compliant PSU unfortunately doesn't have any onboard connectors for case fans which would obviously be no good to you. On that note though, the Be Quiet power supply unit you had chosen will be absolutely fine.