Well, you can probably get a system much better than that now for under 180 quid
What you will need is a processor, motherboard (one that will fit a new processor), and more likely, faster memory (RAM) too.
Now, it depends where your from, but for showing purposes, i'll use Microdirect... I think they are very good for online shopping with!
Of course, if you wanted to go above 180 quid you could, it all depends on how fast a system you want!
AMD Athlon XP 2800 (333MHz) Retail Barton Core inc.fan (3 yr.warr.) = 68.14 pounds (Its not exactly the newest processor, but is definetly good enough. I know a mate with one, and it speeds through games fine)
MSI K7N2 Delta-L nForce2 Skt A 8xAGP +LAN +USB2 +6 chan. audio ATA133 (up to DDR400) = 43.42 pounds (This again is one of the cheapest motherboards I could find, and still have all the functions you might want. I also chose a good make too)
Corsair® Value Select 512Mb DDR PC3200 (VS512MB400C3) = 49.29 pounds (512 mb is the minimum I think for systems to run at full speed and Corsair isn't a bad make either)
Total = 160.63 pounds (see what i mean?)
Now these are just examples, showing you how much better you could do with a short amount of money than the system you have now...
All that isn't a bad spec either, and will definetly run the game and many more, and would last you 3 years or so easily before you might choose to update again, but those are plainly just examples... just have a look around
What Video card have you got though? As this also matters...
Sapphire ATI Radeon 9600XT 128MB DDR AGP +DVI +TV out Retail (This is quite a good card, which would run new games, and is the cheapest I could find a good one for! It's an AGP card, and ATI of course, but that is my prefrence :P ) = 89.24 pounds