I would throw that money into a better psu, another 2gb of memory, and an aftermarket cooler. The stock ones suck. Say, Corsair VX450W £55 Corsair Memory XMS2 4GB DDR2 PC2-6400 £58 (sell the memory you currently have) Arctic cooling freezer 7 pro rev 2 £16 If you're ok with your motherboard, then you still have a GTX460 for £135 and have a bit of money left over
Chances are it is very loud because there is alot of dust build up in fans/heatsinks, meaning they have to run at a higher speed. Give everything a good clean with the hoover. There are many "quiet" fans on the market now, you could swap out some of your case fans for new ones if you want less noise. Also upgrading the stock heatsink may reduce noise under load. Not sure where you will find the backplate though!
Iirc that PSU is 1000W isn't it? Would help if you gave an idea of the games you want to play. As Picky said, a RAM upgrade is low cost at the moment, but that mobo only supports DDR2 and I'm not sure where you'd buy 7200 / 900Mhz RAM these days, you'd probably be better just buying another 4Gb kit. Usually if you contact component manufacturers directly they will send you bits like backplates.
You could probably use any 775 backplate that has 3mm tapped holes. The mounting system looks similar to the titan fenrir
If your going to upgrade to 4GB of RAM, make sure you opt for the x64 bit version of Windows 7, as this is the only one that can utilise more than 3.5 gb (or thereabouts) of RAM.
You should be fine with the windows 7, the difference between the "install" and "upgrade" packages is just the licence to install on a computer without a previous windows licence. What part of the computer is so loud then? the CPU fan? I find that the hard drive is actually the loudest part of my PC, as it vibrates the whole case.
Where is the noise coming from in your computer - is it the CPU cooler? Is it the case? Is it the graphics card? I would echo the comments above really, sell your current RAM and upgrade to 4Gb. The graphics card is still fairly good for older games - so you could keep using it. I'd spend the money on RAM, Windows 7 license (64-bit, Home Premium OEM), the Corsair PSU mentioned above and the cooler mentioned above too. If you have any money left over then maybe you could upgrade the graphics card.
If the rig is running stock then the stock cooler shouldnt be that insanely noisey. Open it up and isolate where the noise is, if its case fans swapping cpu cooler isnt going to sort anything. Are all the fan management options on in bios. Reviews seem to indicate the psu is pretty good and fairly quiet. Not sure id waste brass on swapping that out either.
If you're considering £200 on a case, then the Corsair 800D is also worth a look. Firstly you'll never run out space, secondly if you ever became homeless you could live it!
You should be able to put most of the spare cables into one of the front drive bays with cable ties. Any cables that you are using that are too long, you could loop into one of the bays and out again.
TBH with a little cable management (i.e. stuff them into the drive bays), you shouldn't need a new case - especially as you're already running Notcua fans which are VERY quiet. I'd also like to revoke my previous comments about the PSU, I wouldn't bother changing it. Just get a new CPU cooler and more RAM and save yourself some cash.
