Take a look at the latest "Best Graphics Cards For The Money" article here at Tom's. Here is the August 2010 article at the $200 price point
According to the article, at $200 the best cards for the money are the Radeon HD5830 and the GeForce GTX 460 768GB. The HD5770 is the best buy at ~$155.
If you want to compare to your current card, look at the last page of the article
that includes a hierarchy chart with all modern GPU's. All cards mentioned so far are above the GTS 250 on the chart by 1 tier. According to the author, an upgrade is worthy if it is three tiers or greater, but that is their opinion. The HD5770 is on the same tier as the HD5830, so it may be worth it to you to save the some money.
I suggest you look at benchmarks of the games you like to play and determine if the HD5770, HD5830, or GTX 460 will give you what you need.