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Is this better than a 4850?

yes

It's in the same performance tier. Chart is here:
http://www.tomshardware.com/reviews/best-gaminbaconraphics-card,2544-7.html

It's in the same performance tier. Chart is here:
http://www.tomshardware.com/reviews/best-gaminbaconraphics-card,2544-7.html

FYI
http://www.videocardbenchmark.net/video_lookup.php?cpu=GeForce+GTS+250

It has everything to do with what you're playing.

Take for example a 7600GT and 8600GT. The former is faster at older games but the latter faster at newer ones. An even more extreme example is an old FX5900, when released it did terrible at modern games but was quite good for older ones.

Point being, you can't just look at a chart, you have to seek benchmarks of what you're actually playing and the resolution used too.

Generally speaking if you have to ask about performance on a sub-$100 card the answer is if you really care about performance the budget should stretch a bit higher, cards don't sit in the sub-$100 price range if they have enough performance to fetch more.

Benchmark are only the guide line - you can have a rough idea how it is- If you were serious about certain game/resolution you are playing, you should always look further to see how it runs under the condition you need. You should always check more review/bench tests not one. But if you don't have a rough guide line, how can you investigate/research more ??
There is nothing wrong with all benchmark test, the problem is how people look into it..

To see how well a card like this does against competition it's always best to refer to the most recent gaming benchmarks that such cards are included. This way you are accessing what the latest driver revisions can provide, the resolutions you intend to use, along newer CPU/mobo that don't limit the card This also permits accessing how it stacks up to it more recent challengers. It is always best to check at least 3 reviews, sure they may not be identical results, but at least you should see a trend of how averages with different machines and test methodology.

These include the GTS and 4850.
http://www.techpowerup.com/reviews/Sapphire/HD_5750
http://www.hardwarecanucks.com/forum/hardware-canucks-reviews/24214-xfx-radeon-hd-5750-1gb-gddr5-review.html
http://www.tomshardware.com/reviews/radeon-hd-5770,2446.html

The GTS250 1GB entered into the market to put pressure on the 4850 512Mb, and at the time it did receive a nice drop in power consumption at idle, which made it great competition. ATI countered with their 1Gb 4850, and it turned out as fairly equable 12 round draw after that.