279 will buy you a nice mid-range card these days. before you go and buy anything, you need to decide a few things.
1. does your motherboard support AGP or PCI-Express? AGP is older, and is being phased out. PCI-E is the new standard, and where all the new cards are going. i wouldnt spend any money on an agp card now, personally.
2. How long do you want this to last? The word is Vista will be using directx 10. no cards currently support it, so its a toss up whether you want to buy a good card now or wait until later.
I JUST built my brother a new system, and we used an x1800xt card from Sapphire. I believe it went for about 249. This card kicks ASS, as it is the best rated card in its mid-range class. check this link for the down and dirty:
http://anandtech.com/video/showdoc.aspx?i=2746&p=6
This is a pci express card, with 256mb. Good card, and I'm jealous. If interested, get the 100154L model, as it doesn't come with any extras, and you save money this way.
Crossfire is nothing to worry about. Crossfire is ATI's answer to nVidia's SLI, which allows you to use two cards tied together for better performance. The actual throughput for both of those standards is not a 2x gain, but it is impressive if you have the money to throw at it. You'll need two videocards from the same manufacturer designed to hook together, and a motherboard supporting SLI.