i've used the audiophile 2496 for about two years now and i tell you, it makes a great difference, not only compared with onboard sound cards, but also with some other manufacturers products. of course, has it's disadvantages, but even now, i would make the same choise.

reasons to use it: (or any other non-onboard)
1. m-audio has very good converters digital-analog and analog-digital. among the best in this category of price, bellow 500$.
2. it has very good drivers, asio we're talking. stabile and fast, up to 2 miliseconds latency. very important if you use something else then only audio files.
3. good support.
4. well build cards. phisically speaking - strong connectors, good alignment.
5. finally, we're talking about a different aproach here. without being a real professional card, is much more then a consumer cards, as onboards are. different everything, meant for use in making music in the first place.

good luck!