i'm not familiar with the supreme fx so can't comment fully on your question but I have the X-fi and it seriously blows anything I've ever heard ever before away.
Usually when you read soundcard reviews its like tada this is great bla bla because they you software to test the card but your ear just says its the same.
The X-fi isn't like that though, it makes and unbelievable difference to everything, you can hear every thing. glass sounds like glass, guns sounds like guns etc.
Goto the creative website and listen to the difference between the samples with the xram turned off and turned on and you will hear what I mean.
http://www.soundblaster.com/products/x-fi/demo/
agreed. i have the X Fi platinum and the thing makes my 100 dollar speakers sound like im at the movies.
on the other hand, i cant recommend onboard sound because especially on asus boards, you can sometimes hear static on your speakers/headphones.
keep in mind, ive never hard the sound quality of the striker tho so im not sure if thats a problem on that board, and im taking my experience from all of the motherboards ive had. ive had Gigabyte, Epox, Asus, Msi, and abit, and every single motherboard had the same problem with the onboard sound (static)