okay if you can unplug your drives and then re install them. then use your windows xp disc to restore and repair the registry. Now reinstall any drivers you had for the drives, this should get windows recognising the drives, good luck.
this could be your computer trying to operate safely and it sounds like the tempreture is hitting windows maximum safe temp, it doesn't mean its way too hot but windows doesn't like to operate under those conditions. It could be your CPU tempreture. What fans do you have in your computer? If it...
take out the CMOS battery (cr2032) and leave it out for 5-10mins and then put it back in. This will clear the BIOS. Now make sure all cables are connected and that your monitor is plugged in properly. If that doesn't work, post again.
whats your price range?
If you want cheap then get this: http://www.ebuyer.com/customer/products/index.html?action=c2hvd19wcm9kdWN0X292ZXJ2aWV3&product_uid=48122
Its Crucial so its not bad RAM.
yes go for two western digital raptors and run them in RAID 0. And if you need more room get something like a 200gb Seagate Barracuda. The raptors will make your system run very smoothly.
if all of your used applications can run on x64 then go for it but there isn't really much point at the moment, unless you got it for free, but its not worth trading your 32bit for 64 just yet.
if you have a spare slot then get a 1024 stick that coupled with your 512 will give you 1536 and will give you a significant speed boost in everyday computing. It should also help prevent any bottlenecks.