Many web sites have decent builder's guides that you can use to get an idea of what parts are worth buying and what you can expect to pay, though you have to be careful that the guide you are looking at is fairly recent, since the technology moves fast and prices can change quickly. Sometimes it pays knowing not just what the best deal is today, but what the situation in the market is (like if ATI or NVIDIA is poised to shake up its product lineup in a week).
Here are a couple guides that are reasonably up to date:
Anandtech's sub-$1000 guide: http://www.anandtech.com/guides/showdoc.aspx?i=3486
Anandtech's $1000-$2000 guide: http://www.anandtech.com/guides/showdoc.aspx?i=3497
(Note that those prices include a monitor, audio, keyboard/mouse and OS.)
In addition, you can use resources like Tom's Hardware CPU and graphics cards charts to see how those components (which are the most important to overall gaming performance) stack up. Note that Crysis is not as well-optimized as Crysis Warhead, so you may see higher frame rates in the letter than the former.
http://www.tomshardware.com/charts/
http://www.tomshardware.com/charts/gaming-graphics-charts-q3-2008/Crysis-v1-21,748.html
http://www.tomshardware.com/charts/desktop-cpu-charts-q3-2008/Crysis-1680x1050,818.html
Unfortunately those charts aren't complete, and you may have to do a little digging around to find out where certain models lie relative to the others.
From the buyer's guides you can get an idea of what you want. Go to newegg or a similar site and start adding stuff to your cart, then look at alternative parts in the same performance class (i.e. and see if you can get a little more bang for your buck due to specials or recent price drops). For example, if you see in the guide they recommend a Powercolor AX4830 512MD3-H Radeon HD 4830, you can look at other brands' offerings of HD 4830, as well as see if the 4850 is within your price range, or check NVIDIA's similar-level offerings, like the the 9600 GSO or 8800 GTX.
Good luck.