It all comes down to how much you are going to pay
I would have to say definately DON'T do your suggested upgrade.
Reasons (and probable costs)?
1. Processor. You'll be slowing your processor even with an increased cache you are not going to get the same sort of performance as your existing processor. Ebay has them going for about £33.
2. Memory. This is an area where you could add some more and get a major improvement in performance. HOWEVER you will be paying a premium for SDRAM which you will not be able to use in a future upgrade. 512 SDRAM costs about £85 and you have to ask if thats going to be supported by your motherboard (are you intending 512 as one stick?)
3. Graphics card. Again you are intending to use PCI which has pretty much been superceeded by AGP. You will be paying a premium and not geting the best possible.
I'd guess you are aiming to spend about £200 for a machine you can play games on.
I built a machine for myself the other day (as I had a few components left over from other build and wanted a smaller case one).
I already had:
512 PC3200 Memory, 60G hard drive, CD-Writer, Geforce 4800 graphics card.
I bought the following to make a full system:
1 x IDeq case with Nforce 2 chipset motherboard, built in graphics and sound (£132.60)
http://www.biostar-usa.com/ideqdetails.asp?model=ideq+200n1 x Athlon 2500xp - Barton core (£51.70)
It only comes with 2 slots (1 AGP and one PCI) but since pretty much everything is built in it didn't really matter. The built in graphics are comparible to a Geforce 440 - which is still not too bad for basic gaming system but since I had the card lying around
If I had to have bought the memory I would have been looking at an extra £60 or so. This would have given a total cost of about £240 for a much higher spec machine than the one you are proposing.
If I just wanted to put a basic machine, though still capable of playing games, together I could have done the following (approx prices):
Case + power supply - £25
Motherboard and Athlon 2000 processor (with built in graphics) - £64
Fan for processor - £6
512 DDR 2700 Memory - £60
Hard drive - Use existing
Floppy - Use existing
CD Drive - Existing
Works out at about £155 plus whatever graphics card I wanted to put in (Basic Radeon or Geforce at about £50 mark)
An example of what you could get for £200 mark is:
http://www.komplett.co.uk/k/ki.asp?sku=115294&cks=PRLJust add a case and whatever bits you want to keep from your old machine.