The new Seismic Edge (PSC705) is essentially the same card as the higher-end Acoustic Edge, only without all the I/O bells and whistles. The sound quality should be just as excellent as the Acoustic Edge (uses the same Thunderbird Avenger DSP and drivers). The only thing I don't like about the Philips cards, and this is irrelevant if you have a fairly fast system, is that gaming performance is markedly slower than either of the Creative or Cirrus Logic based cards like the Santa Cruz and Herc cards.
I'd opt for the either the Fortissimo III or Seismic Edge if you're looking for a
budget, but practical upgrade to your existing sound card. I don't have anything against the SBLive, still have mine from '98, but if you're going to upgrade, you might as will upgrade. The SBLive is OLD and was the direct competitor to the MX300. You'd just be trading in one old card for another. The Santa Cruz is a great card too. Have had one for years now as well, but it's a better music card than one for gaming. Also, this card is at the end of it's life cycle as driver support has ceased.
If you really want to spend the money, the best all-around card for both gaming and music right now is the Audigy2. Despite what the anti-Creative zealots will tell you, this Creative card is a real winner and is NOT the same animal as the original Audigy, which I wouldn't normally recommend.
Of course, if you're just looking a plain ol' cheap PCI sound card that doesn't suck, the C-Media 8738 based cards do offer a lot of bang for the buck. This is
my personal favorite, but it's out of stock at the time of this posting. It's a reference design that's used by practically everybody though so you can find clones every where (ie. Herc Muse 5.1 DVD is the same card).