Yeah, don't buy a package that has everything in it. Get a system and buy the monitor, keyboard, mouse, speakers etc seperately. For monitors, dell and samsung are good buys and go for 24" minimum, 1920*1200, best for gaming
I've currently got the razer deathadder mouse and razer lycosa keyboard, both excellent. They are expensive but worth the money, especially the deathadder. I'd go either razer or logitech for keyboard and mouse. For logitech there is the g11 and 15 keyboards and you can't go wrong with the mx518 mouse.
As for the PC itself, spending around the £1000 mark, you'll be looking at either i5 or i7, both equally good when it comes to gaming. i7 is better for other CPU intensive tasks, but both are around the same performance.
Graphics wise, ATi have they're new range of cards out, the 5850 and 5870. Both are excellent, but expensive. A 4890 would be more than adequate for 1920*1200, max out most games, excluding crysis (but I wouldn't worry about that). If you want to go down the nvidia route for graphics, the GTX275 is on par performance wise with the 4890.
Most of the i7 systems will come with 6gb DDR3 standard, which is more than anyone really needs, especially for gaming. Where i5 will likely come with 4gb DDR3, but is definitely more than enough. The memory will be available in 3 speeds, 10666, 12800, 16000+. Unless you plan on extreme overclocking, the 10666 will be more than enough to get a decent overclock from the CPU, and will be cheaper.
Hard drives will depend on how much space you plan on using. I've never went over my 320gb hard drive capacity, but others will be different. I'd choose 2, a smaller one for the OS and one as a back up, storing music etc. So maybe a fast 320-500gb hard drive for OS and maybe games, and another, possibly 1tb for back up. SSD's are still a new technology, they basically have no moving parts, so are silent and much faster than standard hard drives, but obviously are much more expensive. I've used one, it was 32gb, so used it for my OS and a few games. It was much quicker than any other hard drive I've used, but didn't know if it was quite worth the money. I'd personally wait until they become mainstream and therefore alot cheaper.
I'd get a CPU cooler and overclock it, even if it is a small overclock. This will make one of the biggest differences in gaming and doesn't cost anything. Get a decent cooler though to keep the temps down. Watercooling wouldn't be worth the money unless you are an enthusiast, which if you don't want to build the PC I'm guessing you're not. Overclockers do a range of pre-overclocked machines as well, which will come with a guarantee, so there is that option, but overclocking isn't hard, can just be frustrating sometimes :rasp: