In benchmarks the 5770 runs between a gtx 260 and a gts 250, the gts 250 being basically the same as your 9800 gtx+ which means its only slightly better.
You might end up saving money on your power bill if you do this upgrade, also you would get DX11, thats about it.
That being said a 9800GTX+ should run Battlefield Bad company 2, one of the more demanding games out today at that resolution at 60 fps at max settings which should put you at ok.
If you want to upgrade your friend may be wrong, and you should look at the 460, but since you are sitting on the 9800GTX+ wait it out until the 67XX series comes out in 1-2 months. Nvidia said it is planning on dropping the price of the gtx 460 which means it should be cheaper and it would be a true upgrade.
Again, tell us your system specs as a cpu or ram upgrade may be a better option at this point.
Zotac 9800GTX+ 512MB
Intel Core 2 Duo E7200 2.53GHz (OCd to 3GHz)
4GB DDR2 Super Talent memory.
ASUS P5GC MX-1333 Motherboard
I'm on a pretty tight upgrade budget and won't be able to upgrade to anything else that might be coming out.
I was really just looking to grab some small upgrades that would hold me till I can go all out on a new system.
I can use a few references.
I recently played the Arcania Gothic IV Demo. I know its not final product, but its the most recent new game I've tried.
On Medium the game was running 2Xish in a small village with quite a bit going on, and 30ish in more open or uncluttered areas. It's different depending on the game though.
I also recently played through Resident Evil 5 and Metro 2033, and I maxed everything out on RE5. But some games just perform really bad for some reason. Perhaps it's just the game? I believe I had Metro 2033 on low.
And last Final Fantasy XIV. Let's just say that didn't work at all.
I'm also a big MMO player, and when there are a flood of character models on my screen I take a noticeable hit. (Warhammer Online)
30ish is highly acceptable for me, but the majority of the time I can notice FPS drops and lag.
^His board won't support a quad. If he wants to do any kind of system component upgrade, his only choice is a platform upgrade, but at that point he'd almost be better off just upgrading the entire system.