First of all I'd say go with Vista but get the OEM edition which is far cheeper and things like DirectX 10 are Vista exclusive. It used to be that it was illegal to buy an OEM edition unless it came with a PC from the likes of Dell etc but the rules have changed meaning you can get a Full Vista Home Premium for only £75. I know. One of my friends did just that.
Processor wise I'd say go with the X2 4600+
http://www.ebuyer.com/UK/product/122941
It should be perfect for your needs.
I'd say go with this M/board since its socket AM2 like that processor I linked.
http://www.ebuyer.com/UK/product/113843
Gigabyte boards also tend to be really easy to overclock.
Software like CAD and Photoshop and so on are really dependant upon the speed of your RAM. So I'd say go for this.
http://www.ebuyer.com/UK/product/119738
Its 800Mhz RAM which is really fast, not the fastest on the market but it'll handle your needs just fine. I have 1Gb of this stuff and I can tell you it makes a difference. Chances are the RAM in your machine now will be 133Mhz, maybe 266Mhz at a push.
For your graphics card I'd say wait till the new 8 series cards come out from Nvidia. (The only ones out atm are the 8800's which start at £200+ and go up to a good £450)
The other 8 series cards are not out yet but when they do they'll give you the advantage of having DirectX 10. They'll also be a good bit cheeper than the 8800. I've heard the 8600's are supposed to be around £80-100
Also since the M/board is SATA you'd need to make sure your current HD's are SATA or, more likely, if they are IDE. If you have one IDE HD then thats fine since it can be put in master-slave with your drive. Thats what I'm doing. I have one 200Gb IDE and one 250Gb SATA drive.
SATA is faster but slightly more expensive. My 7200rpm 250Gb SATA drive was about £40
All of that comes to around £400 including Vista but not including Graphics card