Okay, so it's just over a week until Skyrim and i'm entirely not confident my PC is up to scratch. Something needs to be done (and fast), to ensure I get the greatest Skyrim experience. So help me if you can.
My current (and fairly old) setup:

Quad Q6600 @2.4Ghz
2 x 2GB DDR2
GeForce 8800GT 512MB
Soundblaster Audigy

So I don't want to have to buy a new system, need a fairly cheap graphics card update (under 100 quid, i.e. pounds) that's going to bring me back up to speed. And I should be okay with the Q6600 right? Or will I get bottlenecking? I've no idea =/
I don't need something top of the line, I just need a cheap and noticable upgrade that'll set me right for the upcoming masterpiece. I've thought about getting an extra 8800 and running SLI, as my motherboard supports it, but I dunno if that's worth it either.

Any suggestions would be welcome. Thanks.

Gts 250, overclock CPU, and you should be fine at medium settings.

Presumably i'd go for the 1GB version? Will anything at a slightly higher cost push me to high settings? Or is my CPU just too old?

And also, I can get a gts 450 for not much more at all, I presume that's worth the extra tenner over the 250?

not worth upgrading to the gts250 imho. i'd recommend saving up some more cash and getting the http://www.ebuyer.com/287106-evga-gtx-460-superclocked-1-gb-gddr5-dvi-hdmi-pci-e-graphics-card-01g-p3-1372-kr. i know its over your budget by a bit, but if you can stand to save up some more cash, that's the route i'd take.

note: i've got no idea about retailers in the UK, i found that site off-hand so you'd be wise to do some shopping around for shops/e-tailers in your area.

Best place to go for advice would be one of these two threads:
1...::THE COMMUNITY TECH THREAD No. 109::. : http://www.gamesas.com/index.php?/topic/1247846-the-community-tech-thread-no-109/
2. Unofficial "Will My PC Run Skyrim" Thread #47 w/ hardware guide : http://www.gamesas.com/index.php?/topic/1249461-unofficial-will-my-pc-run-skyrim-thread-47-w-hardware-guide/

Lots of smart people in those threads that can help you. Try to keep questions and such within relevant threads, if one exists. Helps keep clutter and repetitive threads to a minimum. If you're not sure if there is already a thread, use the search function or PM a moderator.

yep