I have a Guinness setup as well, you see the pics here:
http://i243.photobucket.com/albums/ff180/czwilly/b1.jpg
http://i243.photobucket.com/albums/ff180/czwilly/b4.jpg
It does not matter where you store your tank ( even if it is next to the water heater). When I purchased my mixed gas, the shop rep only had 20lb tanks available. I had the same concearn as you about the gas going bad or separating. The rep told me that the gas will not separate and that there is no shelf life for the beer gas. Besides, even though there is alot of gas there, the kegs seem to be going pretty quick:-)
I now have a keg of Boddingtons Pub Ale and I am loving it!!
Drink up buddy.