Nvidia's GeForce GTX 260 cost around $400 when it first hit online stores, but it's gone down in price quite dramatically since then. Today, Newegg sells an MSI-branded GeForce GTX 260 for only $269.99 with free shipping.
The GTX 260 does lag a few FPS behind the similarly priced AMD Radeon HD 4870 in many games, but let's face it: both products deliver a fantastic gameplay experience across the board, and you really can't go wrong at that price. Newegg even offers a $25 mail-in rebate if you're tempted by the possibility of getting a check in the mail a few weeks or months after your purchase.
Alternatively, those willing to pay a little extra for slightly higher overall performance can grab a VisionTek Radeon HD 4870 for $284.99 with free shipping. This card is also a great choice, and Newegg bundles it with a copy of Codemasters' race arcade/simulator hybrid GRID.