like everyone said before you need to make your own decision, not sure what else we can say. Getting a board that supports pci-exp, is a must right now if you want it future proof, but then you must get a pci-express v/card, 1gb mem is minimum. Keep this in mind, try and find a mobo that has 4 Dimm slots, so if you get two 512mb mem sticks today you'll be able to get more later without having to decide about throwing the old out to make the mem sticks fit. Also a 3500+ is more than enough for any game for the next 2 yrs at least, a 6800 gt or ultra will be able to run any game on full spec for the next yr at least. So worrying over a trifling 100mhz here or there is pointless and a waste of money. You would be better aimed at getting a betting v/card and as much memory as you can, also buying a quality psu about 400w is crucial. One other point, you need to make sure you have a good cooling plan for your pc so many people leave this out. The AMD standard fan is ok, but you need to go with fans and paste that will cool your cpu and case as much as possible, i can tell you as no doubt many here will that having your case COLD, will not only give you a stable pc, but the pc will also be 10-20% faster on average, as pcs work faster colder. This means having exhaust as well as fans that take air in. Try also to get fans with air filters on them to keep the dust out of the pc.
The speed of the HD does not really matter too much as long as it is above 7200rpm, you could go for scsi or sata, but i don't see the point if you have a lot of memory.
I wouldn't really consider getting a sound card unless your a sound affectionardo, as you wont really be able to notice the difference between the onboard sound and a $100 card. Others may argue this point but as i just bought an audigy 2 at $100 i notice no sound quality improvement at all over the onboard ac97.
Dont buy cheap memory, also buy the fast memory you can ie ifyour mobo can support 3200 then buy 3200 memory etc... as (and anyone correct me if i'm wrong) if your boards fsb is 400mhz, and you buy 2700, then you are stuck at running a system at 266mhz. Kingston and crucial are the best buys.
Also when buying a case, don't go for the flashy small ones waste of money, but more importantly you want a case that has space inside for air to circulate and that is easy to work with. The flashy case wont speed up your system and it could cost 50-100% more than you need a basic $40 (pounds) case is fine, just replace the PSU with a good quality one.
Don't overlook a good mouse i think the logitec m500 is a good overall choice, don't have one but heard its very good for the money.
for 1100 pounds you could buy all this and still have money left over. as long as a moniter was not included in that budget. Seeming as you are in enlgand, order from action.com or i think its known as insight.com now or dabs.com, they have some of the best prices, don't buy from shops as they charge an extra 10-30% ontop of what an online retailer would.
As for your last question the L2 cache may make some difference in a couple of years, but i don't think its worth the bother, as long as you but in good quality parts now that work cohesively together you'll find your system runs very well.
Lastly an important note to consider. A computer is in essance a "Sum of all parts". Meaning if you have a 6800GT v/card but the rest of the pc is cheapo slow parts, the v/card WILL NOT run as fast as it should. You need to buy quality parts throughout to get the most out of the system. It would be like you being ultra healthy but you have a weak heart.