AMD processors last as long as Intel ones.  It is a myth that AMD processors burn out and don't last long.  It is likely without o/c'ing it will last many many years, it will last as long as any Intel one.  If you are worried about it over heating and burning out if the cooler fails you are ok, Intel have an on board system that de-clocks the chip to keep it cool, AMD don't use this method but use the bios on the motherboard that cuts the power to the processor if the cooler fails. So no need to worry about it not lasting long or burning out.  I would expect a typical processor age would be no less than 30 years.

As for processors, there is two varieties of AMD 64-bit processors, typically the AMD 64 and the AMD fx.  The fx is the very high performance range, where as the AMD 64 is the more cost-affective alternative.  If money is no object then fx is your processor.

For AMD 64:
AMD Athlon 64 2800 (Socket 754) - Retail (CP-084-AM)      £94.95

AMD Athlon 64 3000 (Socket 754) - Retail (CP-070-AM)
£104.95

AMD Athlon 64 3200 (Socket 754) - Retail (CP-060-AM)
£129.95

AMD Athlon 64 3400 (Socket 754) - Retail (CP-071-AM)
£179.95

AMD Athlon 64 3500 (Socket 939) - Retail (CP-086-AM)
£214.95

AMD Athlon 64 3700 (Socket 754) - Retail (CP-087-AM)
£316.00

AMD Athlon 64 3800 (Socket 939) - Retail (CP-088-AM)
£397.50

AMD FX:
AMD Athlon FX53 (Socket 939) - Retail (CP-089-AM)
£499.95

I recommend using DDR 400 with any of these processors, typical manufacturers:
Kingston
Crucial
Corsair

Crucial is my favorite, i have used them with no problems at all for many years.

I will use crucial as the price example:
512MB DDR 400: £52.99
Dual 512MB DDR 400: 2x£52.99
1GB costs more than dual 512.  Also with dual 512 you can enable dual channel, which will increase the bandwidth of the memory by 2x.

nforce3 is the chipset that the motherboard uses.  nforce3 is currently the fastest chipset available.

nforce3 mother boards:
socket 939:
MSI K8N Neo2 Platinum (Socket 939) Motherboard (MB-027-MS)
£89.95

socket 754:
MSI K8N Platinum nForce3 250 (Socket 754) Motherboard (MB-022-MS)
£69.95

ATX is the size and shape.  AT is the baby format, this is typically for small computers, i would not recommend an AT they often do not perform well, and upgrading is hard. 

ATX is the normal size and is the larger of the two types.  It is not a square shape like AT, it is a rectangle.  I recommend ATX.  I would also recommend getting a good power supply unit from a reputable brand like Antec or similar, with a system like that you must not compremise on the quality of the power, as it could be fatal on the system. 

Thanks a lot guys, that was all very useful.

But I can only go for the cost-affective ones!  My parents earn Zimbabwe dollars which are absolutely worthless so the FX are out of the question!!!!!!  Considering that it would cost Zim$4,499,000.00!!!!!!  You could buy an OK system for that here!  Besides I want a good graphics card, rather than good system and ave card!  Don't forget a descent amount of ram!

Thanks again,
Greg

Thanks,
Greg