If you have a look around the cases section of the site you'll find some galleries of cases. If you don't have a swathe of hard-disks to house then you should be fine with pretty much any mid-tower case. Depends what you like really.
A couple of cases I would recommend and that fall within your budget...
The Lian Li pc7
http://www.overclockers.co.uk/showproduct.php?prodid=CA-054-LL&groupid=701&catid=7&subcat= is a very nice understated case that's very popular. Pretty good cooling with a front and rear 120mm fan, very good build quality and reasonably cheap.
The antec 900
http://www.overclockers.co.uk/showproduct.php?prodid=CA-065-AN&groupid=701&catid=7&subcat= is marketed as a gamers case as it has very good cooling - room for lots of 120mm fans and has a big 200mm fan in the roof. Comes with a side window if that's the kind of thing you like, and blue-led lit fans. To allow all the air to pass through the case it has a mesh front, and doesn't come with dust filters so does tend to suck in a lot of dust.
Both of those are very good mid-tower cases that will happily fit what you need in and cool it fine.
If you plan on overclocking (and it's really fairly straight-forward) then you'll want to go for an intel powered machine since they overclock really well. So either a core-2-duo or a quad processor (q6600) will give you the best value for money and performance.
With respect to your graphics card - depending on the overall amount available for graphics (is the machine being build primarily for gaming?) then the best value in the under £100 category is the x1950 512mb
http://www.overclockers.co.uk/showproduct.php?prodid=GX-064-OK&groupid=701&catid=56&subcat= . Above £100 the sweet spot value-wise would probably be the 2900pro
http://www.overclockers.co.uk/showproduct.php?prodid=GX-160-SP&groupid=701&catid=56&subcat= , since this is basically an underclocked 2900xt, which performs comparably with the 8800gts series of cards. However, you'd be better off asking someone who knows more about it than me, and be aware that nvidia are due to release their 8800gt series of cards soon which will perform better
http://www.overclockers.co.uk/showproduct.php?prodid=GX-038-EA&groupid=701&catid=56&subcat= , though I'm not sure when they're due to be released, but worth holding out for if you don't need to build your machine immediately.
RAM-wise - memory prices are really low at the moment. I would recommend either the crucial ballistix 5300 2x1gb(overclocks extremely well) or the GeIL ULL 6400 2x1gb. Both are available for about £35 for 2gb at the moment.
Power supply - the corsair hx 520
http://www.overclockers.co.uk/showproduct.php?prodid=CA-005-CS&groupid=701&catid=123&subcat= has an excellent reputation and is widely recommended her. It is modular so you only conneect the power leads needed (so less mess inside the case), reliable, quiet and efficient.
Am too tired to cover other components at the moment, will have a go at making a possible build tomorrow.
