Right we've pretty much made a decision, but not what we thought we'd do. After reading all your comments it seems that at the moment it's not worth us upgrading. Mainly because all our software is single core optimised and wouldn't gain much advantage from going multicore. The Sims 2 and Sim City 4 do not use multicores at all, so there will be no advantage to upgrading untill The Sims 3 comes out in 2009, by which time multicore support will be the norm. Also XP is limited in it's multicore supprt, and as we are skipping Vista, 2009 should see a new MS OS.
If we were buying soon however we would have got the Q9450, Asus Maximus Formula X38 M/B, 4Gb or DDR2 RAM, and an ATi HD3870 PCIe 2.0 graphics card. oh and an 800W power supply.
What we are going to do is replace our useless nVidia 6800 GT 256Mb AGP, with an ATi 2600XT 256Mb AGP, and get 2Gb of DDR333 (PC2700) RAM. This is the very best our system can take, and will cost us about £150. Then in 12 months time we will go for the big upgrade, worth about £1,000, of M/B, RAM, Graphics, CPU, PSU.
Thank you all for your help and we have learnt a lot, we'll be reading the forums here in the meantime to try and get completely on the ball with all the new Tech.
Give yourselves a big hand for the help...
PS. Some reasons behind our decisions:
ATi only - We have had 3 nVidia graphics cards and all of them became useless due to driver bugs causing incompatibility with the games we play. We've had 1 ATi card in the past and that just got too old, oh how we wish we had stayed with them. The nVidia's were MX440, FX5200 & 6800GT. The ATi card was a All-In-Wonder Pro 64Mb PCI.
Intel only - We have had 3 AMD CPUs and had one overheat and go pop, on just died for no reason, and one just wouldn't work from new.