You need a 1GB card to run Ultra, 512MB to run on high.
Unless you have pretty beefy single core performance CPU (i3 2100 or better) a decent card should play on high without issue and spending a ton of money on a GPU would be wasted because the CPU will be holding you back. What CPU are you using? Presumably AMD unless you are a running one of those early fancy Core 2 Duo boards (I think, or were those pre- HD 3200 even?)
My pick? A GTS 450 1GB on the high end (hopefully under benjamin - get DDR5 for sure), a card like this on the low end (GTS 250 512 MB ~$30 Shipped after rebate):
http://www.tigerdirect.com/applicati...506&CatId=3670
In the middle ground (price wise) would be something like a used GTX 260 Core 216, but that is big and power hungry in comparison to the GTS 450. It would also need two power plugs. I would advise sticking to something that only needs one (or none!) to not stretch the reliability of your PSU.
SC2 is based on DirectX 9, so even older cards that were decent performers in their day can handle SC2 pretty well.
When I played SC2 I had a 4830 with the rig below and was able to play at that resolution with all high settings and I thought it played very smoothly. The GTS 250 is going to be a decent bit faster than the 4830.
Decent value can be had on the AMD side, as well. A 6750 for ~$80 should be able to run the game at very high settings given your CPU is up for the task.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161379