For mp3s encoded at bitrates higher than 128, e.g. 192 or 320, would it sound better if I listen to the mp3s directly via my soundcard or if I burned it as an audio CD and listen via my cd player/amp?

The reason I ask is because my soundcard is pretty crappy for listening to music (SBlive value), and I'm trying to decide if I should buy a new soundcard or just continue to burn the mp3s as audio cds.

I'd say if your sound card is as crappy to your ears as you say, then definitely go with burning to CD-R and listening over better equipment. When you get a soundcard more geared toward critical music listening than performance/gaming, then you may get more satisfaction out of listening over your PC, and can forego the hassle of encoding to CD Audio and then burning discs.

A lot of people around here can recommend good PC soundcards, with the top two brands for PCI soundcards likely being M-Audio and Terratec. I have a notebook PC myself, and a couple of months ago bought an Echo Indigo PCMCIA card which I'm thrilled with. I use it to connect to my own home theater, replacing two other components: CD player for music (now playing encoded music from my HDD) and DVD player for movies (via my PCs DVD-RW drive). I also plug into friends' home theater systems to provide the music for parties with it as well. The sound card holds it's own against my old method quite well.

For now, simply go with what sounds best to your ears. I know it's more trouble to burn your music to CD-R, but if it's a significant enough difference in sound quality to you, then it would be worth it IMO.

I don't mind the trouble of burning onto CDs, but what I'm really asking if is I'm loosing anything that way? By the way, I'm using Nero 6 to create the Audio CDs.

For high bit-rate mp3s, would it be better to listen on a good soundcard in mp3 format or as Audio CDs?

In theory the quality is the same when listening with a good soundcard or a mp3 capable DVD player.
Burning audio CDs will be more expensive than burning mp3 CDs for a DVD player.

Yes, but the quality of mp3 decoding is very different on DVD players. Cheap DVD players are usually quite bad.
And actually redryder didn't say anything about DVD ;)

With cheap soundcard like M-Audio Revolution 7.1, SB Audigy 2, Terratec Aureon Sky 5.1 or Space 7.1 you get somewhat better quality than basic hifi cd-player.
I get M-Audio Revolution 7.1 myself, and I'm very satisfied.
It depends on your budget, but I'd got definitely for soundcard, like I did. I had SB Live value, just like you have. Tried kX project drivers and many tweaks (all the DSPs off, foobar2000, ASIO output, winamp with MAD input and ASIO output..), but it only helped a little. And I also considered to get cd-player, but all the reasonable cd-players weren't good to my ears (Yamaha CDX-396).

Edit: And with good soundcard you can build jukebox or htpc..

There is no difference in MP3 playback itself. The only difference is in the circuitry (the quality of the analog output).
It would be the same thing if you burned the MP3 to CD, then listen to the CD either in the computer (same sound as playing the MP3s) or in your CD Player (maybe a different sound).