Don't worry about using SLI/Crossfire. One top-end card should do you for now... a mid-range card would do you for now as well.
If you are looking forward to the next gen coming out, I wouldn't splurge too big on a card right now. The new ones are due out in November, so you'd just be turning around and selling your current card at a loss.
Be sure to go with a full size case, though. The new cards are huge. The Armor case Bean mentioned will be fine... it's even the one with the side window.
My Rig: SABLE
Antec 300 Illusion / Antec EarthWatts EA650 650W / EVGA GTX 760 2GB
AMD FX 8320 x8 Black Edition / Gelid Tranquillo / MSI 970A-G43
Sandisk Ultra Plus 128GB / Samsung 840 120GB / WD Black 750GB / WD Green 1TB / WD Red 3TB
G.SKILL Ripjaws 2x2GB DDR3 1600 - Crucial Ballistix Tracer 2x2GB DDR3 1600
Win7 Ult 64-bit - Func MS3 Mouse - CM Storm QuickFire Rapid Mech Keyboard
R.I.P. Danny L. Trotter
... 14 Nov 1945 - 4 Sept 2009