I've checked the specs but they both are about the same, so whats better?

BFG tech Geforce 9800GT

http://www.newegg.com/Product/Product.aspx?Item=N82E16814143154&CMP=OTC-TechReport&ATT=14-143-154&nm_mc=OTC-TechReport&cm_mmc=OTC-TechReport-_-NA-_-NA-_-NA

HIS Hightech ATI radeon HD 4850

http://www.newegg.com/Product/Product.aspx?Item=N82E16814161235

ATI Radeon HD 4850 is more powerful.

and why is that?

The HD 4850...easily.

It beats the 9800 GT in practically every single game and has DX10.1 and SM4.1.

Even the recently announced HD 4830 beats the 9800 GT at practically everything.

so how well would games run with this then? http://www.newegg.com/Product/Product.aspx?Item=N82E16814161260&Tpk=HIS%20radeon%204830

Does very well according to reviews. It too beats out the 9800 GT.

Everyone loves lists, how about this one:  http://www.tomshardware.com/reviews/Best-Graphics-Card,2033-7.html

Everyone loves lists, how about this one: http://internetgames.about.com/od/hardware/tp/tpvideocards.htm

You should get the following 9800GT, not only is it the cheapest but it's new architecture than the BFG.

http://www.newegg.com/Product/Product.aspx?Item=N82E16814130384

For a $100 with rebate on the 9800GT, the minor difference in performance shouldn't be an issue for most.

Generally the 4850 benchmarks higher than the 8800/9800GT, but gaming performance varies depending on the game being run.

But to put it in simplest terms, the performance difference is not anywhere near worth $45