My system is now at least 4 years old, it still gives good performance on LFS, but it is starting to show it's age on other games now.
My system specs are as follows.

Intel P4 3.2Ghz HT CPU
1GB DDR RAM
ATI X800PRO
Intel D875PBZ mobo
3 x 160GB HDD's

Obvioulsy my mobo is still AGP based, so basically, is it worth upgrading anything on this system, or is it better to bite the bullet and buy a whole new system?.
If any of the parts are worth upgrading, what would be better to upgrade?.

You're in pretty much the same boat as me, the only reusable parts are drives, case and maybe PSU. You might not be able to use all 3 drives if they are not SATA, as new mobos seem only to have the single IDE connector.

I reckon you'd be looking at the best part of £500 to upgrade.

I'm eyeing up a Q6600/8800GTS plus associated RAM/mobo and looking to pay about that.

Yeah, if I get a new mobo, then it's 3 new drives, I am pretty sure mine are all IDE drives

I'm re-using my 2 IDE drives and getting a 20 quid SATA DVD drive...can always get new shiny SATA HDDs further down the line

Cash wise your best bet is to ditch the lot and get an entry level Core 2 CPU, 8x00GTS card and 2 GB of DDR2 RAM, coupled with a new mobo.

A 500GB HDD quicker than what you have now can be had for £65

You could be a cheap bastard, throw a cooler on your CPU and overclock it a bit, then throw a better AGP card on it, but it won't last, it will soon be outdated. Seeing as nothing is really compatable with the new gen (DDR2, PCI-E, SATA) I say just go for a build (save your drives, case if you want, PSU if its good enough) Grab an entry Core 2 Duo (E4300, E4400) or a Pentium E 2160. Toss 2gigs of 667 or even 533mhz RAM, throw it on a nice motherboard (eVGA 650i Ultra perhaps?) Maybe grab a nice SATA drive (much quicker then ATA) Expect to spend a more then just 'an upgrade' though...
~Bryan~