Buying a matched pair of memory is generally better than two individual sticks. However in my personal experience as long as it's the same size + make + speed I've never had any problems, just a quid or two cheaper.
If you need to save money and don't plan in overclocking then, 2Gb of Corsair Value Select, PC5300 will be fine at £50 ( Scan code LN12201 )
or
The cheap 1Gb Elixir Major DDR2 PC2-6400, £24.08 each, ( Scan code LN17135 ) which would be fine as long as you don't push it past its specs and you could still get some overclocking from the cpu without going over it's specs.
I must admit to not being a fan of the 8600gt/gts (be aware that the gts is the same chip as the gt just rated for higher speeds) inatial/eairly reviews showed that the 8600GTS was performing around the same as the 7600GT but costing far more.
However now that we are a few driver versions down the line and the price has majorly droped it's better. Still not great but better, performance has improved a bit however it's still short of the 7900gs on most games, although the 7900gs does not have the dx10 capability which the 8600gt/gts does.
As a personal choise I got a 7900gs about 3months back but I don't have vista so dx10 is meaningless to me anyway.
If you want to stick with windowsXP/dx9 then a 7900gs is a great card
I sugest you look for EVGA 7900GS KO Edition on the web site your jpg list came from
it's £90, great card, great price and has the EVGA 90day step up program, so within 3months you should be able to exchange it for a better dx10 card and £90 towards the cost
If you want to go DX10 card then ether try to up your budget to a 8800gts 320mb (£180-£200ish)
or a 8600gts (as long as you're not playing games over 1280x1024 with every thing turned on, ie most 17"-19" monitors will be fine)
For a GTS, 256MB Leadtek 8600GTS, £118.68, Scan code LN18252
For a GT, 256MB Leadtek 8600GT Extreme, £85.76, Scan code LN18257
Sugested the extreme, which is just a factory overclocked card as the basic one is currently out of stock, I'm not a fan of XFX's coolers and the cheap MSI Overclocked only has GDDR2 memory on it.
Although the 256MB MSI 8600GT Passive cooled should be ok £69.21 Scan code LN18329
But in general if you're gameing I'd advoid the 8600gt
EDIT: just a couple things about your List
No hard drive? if you plan on recycleing a current hard drive & cd/dvd then remember that most motherboards only come with 1 IDE socket now which will limit you to only 2 ide drives. Everything has gone SATA now.
No need for Thermal paste with the Arctic cooling Freezer 7 pro it comes with thermal paste pre applied
(so do most aftermarket HS&F)
For a cheap after market Heat Sink & Fan then the Akasa AK-965 seems to be a copy of the AC f7p but a couple of quid less, £12.33, Scan code LN18949
The Scythe Miné, £24.17, Scan code LN13881 is also rated quite highly be people.