(IYO - In Your Opinion)
By ‘sensible’ I mean top spec with standard parts, no over-clocking, no ridiculous cooling methods.
What would you consider the specs of a sensible top spec PC to be these days?
I am sorry if I am not being more specific. I am upgrading mine and want to see if I have chosen adequate upgrade options.
By sensible top spec, you mean a machine that is near the top of the performance curve, but before the prices get really insane, right? All prices taken from www.newegg.com , which IMHO is the best online computer part retailer.
CPU - Athlon64 3000 to 3400. Prices range from $210 to $410, depending on speed. I would go with 3000 - you get 90% of the performance for half the price. Any of the Athlon64 will give you the best performance for your dollar in most applications, especially games - the only Intel processor that can keep up with the 3400 is the Pentium4 Extreme Edition, which cost twice as much. Either buy the retail version that comes with a fan or, look around - I don’t know fans too well.
RAM - 512-1024 MB of PC 3200 DDR-SDRAM. Stay away from the cheap generic stuff- it isn’t worth the headaches. I reccomend Crucial or Corsair brand memory. 512MB of Corsair Value select will run $85.
Video Card - the current Nvidia GeforceFX cards have terrible DX9 performance, and thus I wouldn’t reccomend any of them, unless you could get them for extra cheap, or need the more stable Linux drivers. A Radeon 9600XT is a good mid-high solution, running about $150, or go with a Radeon 9800 Pro for $220ish. Note the the XT comes with a coupon for a free copy of Half-Life2 when that game is released. Just whatever you do DO NOT BUY A CARD MADE BY POWERCOLOR - they are often the cheapest of the Radeon cards, but there is a reason for said cheapness. Stick with ATI built, or Sapphire is another good brand.
Harddrive - Serial ATA drives are good to get if your motherboard supports it, as they are faster than IDE ATA drives, though a bit more pricey. Right now a 7200RPM 120Gb SATA drive with 8MB cache runs for around $105. A similar IDE drive will save you about 10 bucks. If your motherboard only supports IDE, a 7200RPM 160GB drive with 8MB cache can be had for $110. I would reccomend going with Seagate or Western Digital, I have never had any problems with those brands.
Note - I just checked your profile Lobsang and see that you are a UK doper, so your prices may vary, in both amount and currency used. Still, those specs should be pretty good for a almost-top of the line machine.
Thanks for the reply and info 
Unfortunaly I am only replacing the main bits - mobo cpu, and memory. Meaning I still have my geforce 4 card, which as you say is no good at dx9. (but it is bound to be better with the new mobo and cpu) I was aiming for AMD at 3GHz and 500MB of their fastest ram, so it looks like I am going to end up with an OK machine.
My current machine actually has 1GB of ram, but it’s only SDR.
Just a note - AMD uses a PR rating system for its processors, based on clock speed & cache, because AthlonXP&64 processors are a lot faster than similarly clocked Intel processors. You won’t find any Athlon64 running at 3Ghz for a while. For example, the Athlon64 3000 actually runs at 2Ghz and has 512kb L2 cache; the 3200 runs at 2Ghz and has 1MB L2 cache; and the 3400 runs at 2.2Ghz and has a 1MB L2 cache. Still, they can easily compete with Intel processors that are a Ghz faster. The Athlon64 3000 seems to be a a really good spot price wise - in some applications, especially games, it will beat out a much more expensive 3.2Ghz P4.
And the Geforce4 is a pretty good card(unless you have the MX version, which is really just a souped-up Geforce2) - I have a Ti4600 in my computer right now.