You'll need to go 64-bit if you intend on installing a decent amount of RAM.

If you or anyone you know is a student you might be able to get it cheap. Otherwise an OEM 64-bit Win 7 is £75.

To squeeze it in under your max of £750 you could

drop the Asus screen
drop the Antec 300
drop the 8GB
drop the 1TB HDD

add the Dell ST2220M 21.5" LED 1920x1080
add the Xigmatek Asgard Midi Tower
add 4GB Crucial Ballistix Sport
add a Samsung Spinpoint F1 500GB

= £751.80

Alternatively you could:

drop the Asus screen
drop the Antec 300
drop the 6950
but keep the 8GB and 1TB HDD

add the Dell ST2220M 21.5" LED 1920x1080
add the Xigmatek Asgard Midi Tower
add the Sapphire ATI 6870 1GB

= £734.82

where is this from?

Overclockers

They're fairly popular products though so you could shop around for best prices and/or simply avoid OCers if you don't like them (which some people dont).

having finally managed to resave a bit of money after having to buy a new gearbox, this is now back on the radar... is now the time to buy or should I wait for a couple of months?

are the previous options still valid or is there new stuff now available?

Prices look ok at the minute. Where I previously quoted a Samsung 1TB HDD for £95 you can now get a WD 1TB (single platter) for £75. The 6950 graphics card is also available cheaper now e.g. Club3D 6950 2GB for £176 (or GTX560Ti 448 edition for about the same money).

Prices from Overclockers again.