Sadly you can’t run your GPUs through the likes of an RPi, if you could I’d of been doing it since day one. :P
You’re in the same situation as me it seems, except I can only run for 5-6 hours a day.
There’s been a lot of talk of the windforce 7950s being the favourite card to pick ,due to thier cooling abilities but aslong as you have good case airflow you could get something like :
XFX Double D Radeon HD 7950 3GB GDDR5 Graphics Card [FX-795A-TDJC] + 4 FREE GAMES!
Aria: http://www.aria.co.uk/SuperSpecials/Other+products/XFX+Double+D+Radeon+HD+7950+3GB+GDDR5+Graphics+Card+[FX-795A-TDJC]+%2B+4+FREE+GAMES!+?productId=52185
£225, sell the games code for around £30-50, makes them £195-175 ea.
SAPPHIRE Radeon HD 7950 Boost Edition 3GB GDDR5 Graphics Card [21196-00] + 4 FREE GAMES!
Aria: http://www.aria.co.uk/SuperSpecials/Other+products/SAPPHIRE+Radeon+HD+7950+Boost+Edition+3GB+GDDR5+Graphics+Card+[21196-00]+%2B+4+FREE+GAMES!+?productId=55010
£226, sell the games code for around £30-50, makes them £196-176 ea.
Otherwise, take to eBay. None display cards (faulty) sell for £100-120 a pop. Could drop lucky and be perfect as a headless mining card. And used/working cards sell around £180 - 220,
If you’re not to confident you can run your cards cool with the stock coolers then opt for the Gigabyte windforce.
(Note, my 24/7 rig will soon have the 7970s + 6 x 120CFM fans in a regular ATX case and that should be sufficient enough for me to run them cool and 24/7)
Edit: From experience, the XFX DD (I have 7970s so slightly more heat) stay around 86C with the fans @90% and ambient approaching 32C where the current rig is and has been running awhile.
They are running inside this rig with 2x 120CFM Scythe Jyuni fans pointed at them at present:
http://forum.feathercoin.com/index.php?topic=137.msg1160#msg1160