Well, I can't answer you regarding the difference between on-board sound vs. Soundblaster, but you always want hardware acceleration, whether it's for sound or video. Software acceleration is found in cheaper boards because it uses your CPU to process the sound. The manufacturer saves on the cost of an on-board processor. This makes your machine slow down a bit because processing sound is very intensive.
Hardware acceleration utilizes a processor on the sound card to produce the sound, thereby freeing up your processor for more important things. Not being a real audiophile, I use my on-board sound (C-Media). It's more than adequate for what I'm doing and for listening to music. Most low end computer speakers aren't very good, so you'll get more of a boost if you buy really good speakers. They start at around $100.00 US and can go to $500.00.
I bought a pair of Bose Media Mate speakers for $99.00, which did wonders for my sound. There are of course better speakers, but these little beauties are good for desktop use where you don't have alot of space. They have amazing bass for such a small unit. My wife even likes them, and she hates computers.
Sorry to get so long winded, but thought you would like the info. Buy the best speakers you can afford and go with hardware acceleration. Soundblaster is a good name in audio, so you really can't go wrong with it. If your Asus board is fairly new, I'd look up the specs and compare the sound chip's specs with the SB board. Chances they're fairly close.