Originally Answered: Upgrading graphics card-suggestions?
Argh, that power supply throws a monkey wrench into your whole upgrade plan.... These days even a 300W power supply is sub-par. With just 250W you don't have enough for any entry-level gaming cards. You'd have to drop down to non-gaming budget cards- which are a big improvement from your integrated graphics, but much slower than you'd want.
Even that bargain 7600GS requires at least a 300W power supply to run (the official specs say 350W, but the published numbers are often a bit inflated).
Your best option is spending a little more, so you can upgrade both the power supply and graphics card. Here's the combination I'd recommend:
That power supply is strong enough for anything up to a Radeon 5770 (roughly $150 level cards). It's also a decent brand that shouldn't burn out with 48 hours of opening the box, as is often the case with bargain-basement psus.
The Radeon 4670 is the best bang/buck you'll find below $100. You could save $20 by going with a Radeon 4650 or GeForce GT220 instead, but there's a big dropoff in performance. While even those $50 cards would be like night & day compared to your integrated 6150se, when such a small amount of money almost doubles your fps in games, it's worth spending.
Re: general evaluation of graphics cards, you best resource will be online review sites like Tom's Hardware, Hot Hardware, Techspot and Xbitlabs. Because ATI's and Nvidia's technologies are so different, you can't accurately judge how good cards are simply by looking at underlying specs like core speeds, memory, stream processors etc. Some GPUs are faster than others, some design approaches work well and others don't. Some cards look good on paper but underperform in reality. By reading reviews you'll see how different designs actually performed in game titles.
Yes in general bigger numbers are generally better when it comes to core and memory speeds. More stream processors are better, but that's primarily useful for comparing designs from the same company (i.e. ranking different ATI cards against each other, or differemt Nvidia cards against each other) It doesn't work comparing Nvidia vs ATI because their techologies are different and they use stream processors differently. On average, ATI cards have 5-6 times the number of stream processors as competing Nvidia cards.
GDDR2 cards are low-end, much slower than GDDR3 or GDDR5 cards. Having a 256-bit interface is usually better than 128-bit (all other things being equal). But the GeForce 9800GT is a 256-bit GDDR3 card which performs slightly worse than the Radeon 4770, which is a 128-bit GDDR5 card. It turns out the faster memory offsets the lower memory bandwidth. But the Radeon 4850 is also a 256-bit GDDR3 card... and it narrowly beats the 4770.
Just looking at specs is kind of like trying to buy a car based upon the number of fuel injectors, rather than reading Consumer Reports to find out which models did better on acceleration and braking tests.
So reviews/test results are very valuable- I presonally prefer take in-game results over benchmark suite numbers any day. Some cards score wonderfully on Passmark, but crawl in Call of Duty at 1680x1050.