The GTS 450 uses DDR5 ram while the GTS 250 uses DDR3. This mostly makes up for the smaller bus width but more importantly the GTS 450 has a more powerful GPU that can also be overclocked to much higher levels.
It's a good card for low/medium resolutions. If you are using a high resolution monitor(1080p or above) I would recommend spending a bit more for a GTX 460. If you are in the US this one is $140 after the promo code and $120 after the mail-in rebate;http://www.newegg.com/Product/Product.aspx?Item=N82E16814121390