I went with the Asus TOP 4890 1gb card. It has a factory OC at 900mhz Core and 1100mhz memory. I have never seen it go over 46c under load (playing WoW, TF2, Crysis) in my Antec 1200 case. I am very happy with this card. Supposedly, it can be overclocked further to 1ghz on the core with voltage adjustments, and comes with software to do so. I have NOT attempted this though. I am very satisfied with its performance as is.
^ This is the link to a 1gb vs. 2gb comparison on the Nvidia GTX 285 cards. Yes, I know it's not the 4890s, but I would imagine the results would be the same. At this point in time, 2gb cards are just a marketing gimmick, imo. This bench shows minimal, if any, gains in FPS using the 2gb model.
Just my .02
EDIT: I tried to locate the Asus TOP 4890 on newegg for you, but it seems to not be listed there anymore .
I went with the XFX 4890 1gb. I think it would depend on how tight your budget is. You can spend 20/30$ more for the higher bins factory stock OC at 900/1000+ and can OC higher than that if you really wanted to.
But it also depends on the resolution you're playing at. I'm running 1680x1050, and there's no game that i have that this card can't handle on max. (Tf2,L4D, Cod 4/5, Bioshock, Aion). Mine came at the stock speed of 850/975, but i easily OCed it to 900/1000 in a matter of 30 seconds using ATI CCC. I had no need to go higher because i was already getting 60+ on every settings maxxed out on those games.
XFX also has lifetime warranty and you're pretty much set to be able to OC it pass what it can do stock very easily.