It is fantastic so far; the speed is great. My only problem is that the onboard graphics do not seem to be enough for some games. I am not a gamer so I don't need anything really expensive, but I'd like it to be able to handle graphics better than it is now.
My motherboard does not say that it supports either SLI or Crossfire, so I'm assuming neither. Does that limit me in what card I can choose? It has one PCI-e 2.0 x16 slot and two PCI-e x1 slots.
The 5850 is still a nice deal at $150. It is however overkill for your current resolution. Fortunately that Sapphire card has a lil brother in the HD5830 which is also crazy cheap on newegg and this time it is in stock; http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Still a bit of overkill to be honest but nothing else makes sense with it selling at that price. For example it's cheaper than the cheapest HD5750 you mentioned and at your resolution it is approximately 35% faster.