Never compare number of shaders, clock or memory speed between cards that are not the same generation.
The only thing important is the performance they offer. Memory bandwidth and processing power might be an indicator.
You cannot compare radeon and geforce in terms of shaders, clock or memory. There are different architectures involved. They do pretty much the same thing using different methods.
In the end you are interested in what it can do not how it does it.
Just see which one achieves the highest most consistent framerate at an affordable price.
Read a few reviews and watch the frame rate charts.
You can find them all here and on other sites.
http://www.tomshardware.co.uk/review/Components,1/Graphics-Cards,4/
Search benchmarks for
Radeon HD 4830, 4850, 4870, 4850x2, 4870x2
Nvidia 9800GTX, GTX 260, GTX 280, GTX 295
Decide which one of the cards offers you the best performance for what you can afford and buy it. You might want to read 2-3 reviews entirely, you will start to understand things a lot better. After that just check the frame rates, power consumption and price.
4830 < 9800GTX < 4850 < 4850 1GB < 4870 < 260 < 4870 1GB < 260 (216) < 280 < 4850x2 < 4870x2 < 295
Pick one card and then check all it's variations... for factory overclocks, cooling solutions, sound generated accessories and prices.