What should I upgrade or replace to make it better? I have around £600 to spend plus what ever I can sell the old parts for. I mostly use it for High end gaming, but in some newer games my PC doesn't seem to play well on high setting anymore.
Also the Nvidia Geforce 9800 GX2 is running quite hot, right now just writing this, it is on 74 °C (163 °F), is that an normal temp?
If your GPU is at 74 degrees in 2d mode, yeah, thats warm. If its having to throttle back because of heat, then you'd see lagging in game. What case do you have? Have you monitored fan speeds and settings at all?
Honestly, the system you have is quite good for gaming. While you could buy a new platform, I'd hold out. If there was an issue, it'd be with just the mobo using a pciex16 slot instead of the pcie 2.0 x16. Something as simple as this would allow you to OC your CPU, not bottleneck the GPU, and be cost effective: http://www.dabs.com/products/gigabyte-s775-intel-g41-dd...
Get an aftermarket cooler, and that CPU should OC to 4ghz easy. That will be a nice bump in performance for you.
Windows 7 - I run dual boot XP/Win 7 and the gaming in Windows 7 RC is superior to XP. Faster, better looking. Definitely something to consider.
The reason I'm not saying go for a full platform upgrade is, you seem to be willing to upgrade every other year. And with a budget like you have, I'd say step up on the other parts that can make a big difference while that CPU and GPU still have it. What the bulk of your budget can go for and see some great results, is an SSD. Now I'm not the one to say this is better than that, but you can spend a couple hundred £s. But in 2 years, you'll have a SSD and can spend your money on a platform GPU update then.
SSD - ~£300 (educated guess)
Thats £500 and will get you to your next upgrade nicely.
Thanks for all the help guys . The Resolution for my monitor is "1680x1050", and the case is a Coolmaster cosmos 1000 Link, I think I'll move a few of the fans around to get more air flow over the GPU, as Everest says the case is 42 °C (108 °F).
Also thanks for the tip about Windows 7 I think I'll get that. and I'll look in to Overclocking my CPU.
But I think I'll stick to what hardware I have for now.