If you wanna stick with Nvidia you are going to have to take a lesser card for more money. They really don;t have anything great in your pricerange. the 550ti sucks and you are above the gtx 460.
PC CANADA - CP AX6870 1GB GDDR5, RADEON 256 BIT, DL-DVI-I/ SL-DVI-D HDMI/2XMINI DISPLAYPORT [AX6870 1GBD5-M2DH] - The Original and Highly Rated All-Canadian WebStore price match that to NCIX and you get $203 with tax, unless you go second hand it isn't going to get much better than that.
It's a powercolor HD6870.
or for a bit less you can get this
NCIX.com - Buy XFX Radeon HD 6850 775MHZ 1GB 4.0GHZ GDDR5 2XDVI Display Port HDMI PCI-E Video Card - XFX - HD685XZNFC - in Canada
If you really wanna stick with nvidia you should get
NCIX.com - Buy MSI GeForce GTX 460 Fermi Cyclone OC 725MHZ 768MB GDDR5 Dual DVI Mini-HDMI DX11 PCI-E Video Card - MSI/MicroStar - N460GTX Cyclone768D5/OC - in Canada
Both radeon cards are better than the nvidia but the nvidia will cost you less (honestly I would take the ATI).