Just checked Newegg USA. No HD57xx in stock. Other HD5xxx are overpriced compared to performance of HD7xxx cards.
My single HD7970 beat my two HD5870 in CrossFire and crushes them in "compute" tasks. Also handles 3 monitors for Eyefinity with a single card.
The HD7750 in our HTPC performs as well as an HD5770 (we own one) -- which was all AMD wanted it to do.
For a normal sized (1920x1200) single monitor, HD7970 is overkill for a person on a budget.
So, suggest get the best HD7xxx card you can afford. On sale, HD7970 can be had for just under $400 with free shipping -- just check Newegg every day until you find one you like for that price. They had a dual fan model a few days ago at that price. Today they have a couple standard cards for $410 with free shipping. Assumes one lives in USA.
GTX670 is a very good card for the money. Stock HD7970 about the same but wins BIG in compute tasks (many applications are starting to use the GPU compute to accelerate the application) and also wins for multi monitor.
No hurry of course. Prices seem to go down and speed goes up every few months. HD8xxx series not due until December at earliest.
Crossfire 2x Sapphire HD7970 OC - Eyefinity 5760x1200 24", 1xDell-U2410 H-IPS + 2xHP-ZR24w S-IPS, Ceton InfiniTV 4, Piledriver FX8350@4.6GHz w/Swiftech Water Cooling, 16GB GSKILL PC3 @1866, ASUS Sabertooth 990FX, OCZ Vertex 4 256GB SSD, 4x Seagate + Hitachi + 2x Sg Ext = 14TB, Lite-On DVD, OCZ ZX 850W PSU, CoolerMaster HAF922, TM Warthog HOTAS, TM T-Flight Stick X, TM Cougar+FSSB & CH Pedals, Saitek X52 Pro & Pro Combat Pedals, TrackIR5 w/TC Pro, Windows 8 Professional 64b