EVGA GTX275 SuperClock vs. Sapphire HD4890 Toxic

Trying to decide which of these cards would be better performing in single and dual card configurations. I would most likely buy one card now and the other one in a couple of months or so. The build it would be going in is:

Intel Core i7 920
Xigmatek Dark Knight Cooler
Gigabyte GA-EX58-UD4P
CORSAIR CMPSU-850TX 850W
Western Digital Caviar Black WD6401AALS 640GB x3 (RAID 0 or 5)
COOLER MASTER HAF 932 RC-932-KKN1-GP

The two cards that I am looking at are

EVGA 896-P3-1171-AR GeForce GTX 275 Superclocked Edition 896MB 448-bit DDR3
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

SAPPHIRE Toxic 100269TXSR Radeon HD 4890 1GB 256-bit GDDR5
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

This is my first build so I really do not have any games in mind yet, but anxiously awaiting jumping into some of the hard core FPS and such. I will also be doing some general use and developing (VS 2005/2008).

I don't think heat will be much of an issue with the case selected, and I think I could always install better more efficient fans, correct?
Also, is it absurd buying into these factory overclocked cards? Would it be better to buy factory stock clocked cards and then "try" to overclock them then? If so, then what card would be the better card (I know they trade blows pretty evenly) - based on current sales, brand/warranty, etc..?

Thanks for the advice!