I bought my ATI Radeon HD4670 last year for 90 US dollar and I do have to say that it is a very nice card for that price. It handles my favorite race games ('Race driver: Grid' and 'Need for Speed: Shift') beautifully on a resolution of 1920x1080.
Now I have to assume that the prices for that range of card has dropped since and since the new 5000 range is just out, there have to be some nice deals on the net from stores getting rid of old stock.
The reason I mention my card is that if you have sudden need to play a game, this card will do it nicely.
Now I do not know if support for DirectX11 is a requirement for you. If so, then you will have to buy an ATI in the low 5000 range but I'm not sure about being in compliance with your budget. Don't worry though, you will not be spending that much more. I saw today in a Dutch news article that Asus has an ATI HD5750 based card for the retail price of 120 euro's. That means that web-shops will be able to deliver it more cheaply than that.
NVidia promises DirectX11 support with their GT300, but that card is not out yet and will cost a lot more than your budget allows.
There can be a problem popping up though. New video cards are long (size wise)....really long! Depending on your PC case you can have some trouble installing a new video card.
My advice would be to install a card that supports DirectX11, Windows 7 comes standard with it. One of the tricks that DirectX can perform is Tessellation. Now I saw a demo from a game engine
with that effect...it is awesome!