Replacement GPU

Hi all,

I need help selecting a new GPU to replace my old one (specs below). Last night, while playing Lost Planet 2, I started having significant artifact issues.

I built this machine a few years ago for FPS games but it has started to age. Over the past year or 2, the system hasn't been challenged much at all, as I played mostly WoW and SCII on non-challenging graphics settings (when I wasn't too busy with work to play). However, I am bored with those games and have started playing other games and, well, the card is showing it's age.

So....I am looking for a reasonably priced GPU (or 2) to get me through the upcoming games (definitely Diablo III and possibly BF3....who knows what else) and years. I don't have a hard-set budget, but I don't want my wife to flip when she sees the bill.

I would like to try to get another 2 years out of this machine, but I realize it is aging...is it even worth it? I could probably rebuild if it came down to it.

Any advice? I know a lot of people are really stoked about the 560 series cards, but how do they stack up to, say, a pair of cheaper cards?
I generally look at nVidia cards because Radeon pissed me off years ago) but nVidia's awful support has been testing my patience lately and I would consider a Radeon again.

Also: my HDTV is hooked up to my computer and most everything we watch is run through my comp (streamed sports, HBO, etc.)

Sorry if the post is a little bit rambling and disorganized...I'm not good at writing.

System Specs:
Motherboard: GIGABYTE GA-X48-DS4 LGA 775 Intel X48
CPU: Intel Core 2 Duo E6850 Conroe 3.0GHz LGA 775
RAM: 6GB OCZ Reaper (1066 DDR2)
GPU: EVGA GeForce GTX 280 (1GB 512-bit GDDR3 PCI Express 2.0 x16)
Heatsink: ZALMAN CNPS 9700 NT
PSU: Antec TPQ-850 850W
HD: Seagate Barracuda 750GB 7200 RPM (SATA 3.0Gb)
Case: APEVIA X-CRUISER-BK Black Steel ATX Mid Tower
Sound Card: Creative Sound Blaster X-Fi XtremeGamer 7.1 (PCI)
Plenty of fans and air-flow

Thanks again for any help,
-Slow Hand