Hey Jergens,
what your suggesting is not possible you cant have a ATI card and a Nvidia Card in the motherboard like that. There is only two ways it works you either have Crossfire Configuration or a SLI Configuration. as far as im aware of.
Crossfire is - ATI Dual Video Cards
SLI - Nvidia Dual Video Cards
your GTX 460 im taking is the 1gb oced model? honestly man the 6990 is more then enough for any games you want to play. Dual video cards to me is a bit over the top. its only adding more heat and power to your system having dual video cards, but thats just my opinion.
the difference you will see from the 460 to the 6990 is going to be a huge performance increase. Below I have listed a website I use to compare graphics cards. i have compared the 460 to the 6990 and as you can tell there will be a massive performance difference.
http://www.hwcompare.com/9553/geforce-gtx-460-1gb-vs-radeon-hd-6990/
I personally am upgrading as well from a GTX 460 to a 6950 i was going to get the 6970 but there can be rumors and finaly been confirmed that you can overclock the 6950 to a 6970s specs. so basically you will be running a 6970
and the best thing is i save myself $200 instead of paying $400 for the 6970 I pay half of it and i get the same product in return when i overclock it
and btw yes you should give the GTX460 to your brother that card is still a decent card ive had mine for at least a good 3 years and havent had any issues with it. and also i would think your a top brother if i was your brother haha.
anyways man Good luck with everything