Don't go with an e-machine. They are a rip-off. In most cases, you can't even upgrade them as their mobo is not replaceable.
Further more, the specs on the one you mentionned are so-so.
The sound card 9600 PRO is a good card but the 9800 is better.
If you can, try and get a SATA HDD (150 vs 133) instead of the ultra IDE listed.
The ASUS P4s800 comes with a very decent integrated sound board so you don't really need another one. Here it is listed from the ASUS site:
ADI AD1980 SoundMAX 6-channel CODEC
S/PDIF out interface
here is a link to the ASUS board specs: