Waiting for Skyrim and BF3 would have the added benefit (besides actual benchmarking of those games to know exactly what cards perform best) of possibly giving you a glimpse of what the incoming cards will look like --
AMD at least looks poised to launch some new stuff later this year, perhaps they'll manage to shake up the market a bit as early as this fall.
Personally, I'm hoping for an earlier mid-range 28nm process pipe-cleaner, more so in fact than seeing what top performance the high-end will bring.
On the other hand, if that 4650 can't run games you want to play
today, I really don't see why a trickle-down style upgrade across both machines wouldn't be just what the tech ordered. I'm fairly comfortable in approximating that going from the 4650 to the 260 would increase FPS in your brother's system by about 2.5x. Turning unplayable to playable.
You would, as Happy Medium said, need to check the PSU of that computer.
On your side of the upgrade, switching up to a GTX460 1GB (non-SE, of course) would increase FPS from the GTX260 by about 40% (?? -- if somebody reads this and disagrees, please chime in to correct my fast and loose figures

) costing you about 130$AR, but keeping system load power consumption squarely within the same territory as your previous configuration, as
evidenced here on Anandtech.
So, summing up, your bro's system gets lifted up into playable framerate territory immediately, even at 1920x1200 high-res, though he may have to spend some scratch for a better PSU, (say 50$ -- maybe 75$, for a really nice unit which will last him well into another, future build) and reimburse you for the GTX260 (prices on FS/FT right here on this site would place the value of your used card at about 65$ FYI) so
at worse, factoring a new PSU, that would amount to a 125$-ish upgrade for your bro, (50$ at best with you cutting him a nice deal on your old 260)
And
You would upgrade to a GTX460 1GB, which is arguably the best bang-for-buck card ATM. And which, factoring in the return on your 260, would cost you well under 100$, closer to 75$ in all likelihood, and keep your system power needs within the constraints of your current PSU. In fact, if you look closely at the
page I linked earlier, you could move up to a 6850, 560, or 6870 safely.
Which, getting back to your original question, you wouldn't have been able to do by adding in a second 260, assuming that HP box would indeed even have allowed such a move.
Need I say more? 'cuz I'm getting pretty tired of typing, frankly
