
I doubt he'll be wanting to keep his GF1 card and just upgrade his PIII 1ghz as for gaming the gfx card is way more important. What's your budget
nagibbs? Do you mind trying a little o/c'ing as a lot of chips have a lot of headroom in them by the very nature of their design?
Pete, the Rad9000 is seriously slow but the Rad9000PRO is much closer to a Rad8500LE (in turn 10% slower than Rad8500). The Rad9100 & 9200 are more like Rad9000
PRO with useless AGP8x aren't they? There's also the Rad9200PRO which should be close to Rad8500 speed, right? If you don't want to spend a bucket you'll have to go AMD so here's an idea of what to get, price it up at Newegg?
TbredB (must be TbredB not A or Pally) XP1700+
TbredB (must be TbredB not A or Pally) XP2100+ (useful for mobos lacking multiplier/freq adjustment due to 13x133)
Barton (always uses Barton) XP2500+ (Barton is 5% faster than TbredB clock for clock)

All three of these should get around 2.2ghz meaning TbredB equal XP3000+ (roughly) while Barton equal XP3200+. You should be assured (as you can possibly get when o/c'ing) of at least 2.1ghz which is still seriously fast. Heck even stock the lowly XP1700+ is HUGELY fast compared to PIII 1ghz and able to get great perf from gfx cards and lovelly perf in games. You'll want a decent HSF, the inexpensive speed adjustable Volcano 11 is an excellent choice.

256MB PC2700 is okay but PC3200 is better and costs very little more. A good brand like Crucial or TwinMOS are great without being expensive. 512MB is nice but seriously not needed esp if your budget is tight.

Mobo choices come down primarily to chipset. SiS746FX should be cheapest but lack o/c'ing options, leaving you with FSB to o/c only and the official max is 333FSB. SiS748 simply offer 400FSB. KT400 (KT400A slightly faster) only officially offer 333FSB and many entry level ones may lack multiplier adjustment too. KT600 are KT400A with 400FSB, even entry level ones should supply multiplier adjustment though. nForce2 come in a variety of flavours but basicly all should run 400FSB and have great o/c'ing abilities. The Dual Channel DDR ones do give a 5% boost which is nice but not overly necessary as without it they are still at KT600 speeds and it raises the cost as you need 2 sticks of RAM. Teaming an XP2100+ up with a mobo lacking o/c'ing options like multiplier adjustment is a good idea as the 13x default should not be limiting even on a mobo lacking 400FSB ability, plus 1.6v is default IIRC and should allow enough headroom. The XP1700+ DLT3C come with 1.5v which will be limiting if your mobo can't up the voltage, XP1700+ DUT3C come with 1.6v and overcome that problem, the 11x multiplier should allow a 400FSB mobo achieve the magical 2.2ghz even if it lacks multiplier adjustment. XP2500+ should always come with 1.65v and again an 11x multiplier so dito to the DUT3C XP1700+ above. So lack of o/c'ing options shouldn't be overly problematic meaning you can get an entry level mobo but do ensure it can do 333FSB or better still 400FSB. nForce2 is best if you want an easy life but expect to pay more.

Gfx card. GF4TI4200 is great for the money and I'd suggest not going lower as your PIII 1ghz will most likely be close to getting max gfx perf out of most of the slower cards. Rad9600PRO and FX5600ultra are 50% more pricey but give you better AA+AF and DX9 so are more future-proofed.

I imagine £ will translate closely into US$ since we pay a fair bit more over here and have 17.5% VAT. So prices wil be...
£40-70 CPU capable of 2.1 - 2.3ghz (add £15 for HSF)
£50 256MB PC3200 good brand
£35-85 mobo
£85 GF4TI4200 128MB
TOTAL £210 - £290
PS. As a side note you may find your case isn't up to these CPUs, should be fine but may limit achieving the highest clock speeds. Your PSU is unlikely to be good enough although you could buy the stuff hoping it will do and then buy a PSU if you need to. 350W would be a minimum ideally from Antec or Enermax although Sparkle and ToPower are also good and cheaper, 400W is wise if you go for an unknown brand. Other than that there should be no unpleasent surprises and you should find you can easily stay under £300 regardless, you could always get the 4200 now and look to save £200 for the other upgrades.