Ebay grrr?lol.
Ok take a look at Amazons website.
Now intel
PSU are pritty expencive so you could choose amd DUAL core.A lot of mainboards are compatable with intel and AMD CPUs.
Take a look at my PC specs under my name on what I recently built.
Checkout some of the threads wich others are building.
You can buy a case for £15-£30 or upwards as long as it has decent scope and space inside and gives ample space to add extra fans for cooling.
Try ATX case.
You can get a decent SLI mainboard for maybe £60-£80.
CPU can cost around £80 upwards for dual core.
A lot of Mainboards newer ones take an average of 8-16 gig
RAM.
You can get a OCZ stealth extreme 600 watt
PSU for about £45 upwards.Or get say 700 watt.
Do not buy cheap on power supply unit or
RAM.
If you are sticking with XP 32 BIT this will only register you have 3.25
RAM even though you may have more fitted in there so I would go for a 64 bit OS.