Should I just get a 4890 instead of 5850?

If you can get a 4890 for under $200, then go for it. Otherwise, it's more worth it to shell out $270 for a 5850.

A 5850 is about 5-10% better than a GTX285. And a GTX285 is about 10-15% better than a 4890.

DX11 won't be implemented for a while, so getting a 4890 will hold you until DX11 matures and the prices of the ATI and Nvidia DX11 cards drop.

(or get a 4870 for about $150)
 
stupid ATI. This is definitely not a demand problem. These things have been out of stock since they came out. Obviously they can't produce enough. The 3 biggest canadian computer part retailers haven't had them in stock for 3 weeks now. It's not like they come in and are sold out. They just don't receive any shipments. ATI shouldn't have launched if they only had 500 cards to sell...
 
I would say go with either 4870 crossfire or 4890 crossfire depending on how much you could get them for. Either crossfire setup is a lot faster than a single 5850.

I built a 4890 crossfire setup and I'm very happy with it. Just make sure you have a powerful CPU to support your video card(s).