24 bit cards usually aren't standard. Most cards just do 44khz 16 bits.
When you plan on listening to mp3's through your computer speakers it's never worth investing in an expensive soundcard. You won't hear the difference.
More than 4 gigs of RAM is probably a waste of money unless you plan to do a lot of photo/video editing. I am running win7 with 2 gigs of ram on my office PC now and that's just fine. I will upgrade to 4 sometime, but more won't be neccesary.
I do a lot of photo (photoshop/lightroom) and video (premiere pro cs3) editing on my audio/video PC and even then I don't often run out of ram. That PC only has 4 gigs.
As for harddrive space: 1,5TB disks are pretty cheap now. I suggest you have one of those. But please buy a second one (maybe external) to backup your ****. Raid could be an option too, if your motherbord supports it (when you put two identical drives in RAID mode, you only see one drive, but all your files are on both drives. Whenever one drive crashes, all your files will be on the other one
).
Other than that: Avoid total budget stuff with SiS chipsets and so on.
Every 'better' brand will be just fine.
You should probably be able to stay under $500, screen included, should you want to.
But I don't know what you want to do with your PC. Don't spend a lot of money if you're just surfing, watching video's and listening to music. A PC for less than 350 dollars can do that just fine.
My PC's have specs from 2007 (not all PC's are that old, I often buy old hardware new because it's dead cheap then). They run brilliantly.
Most of the time, when your PC is less than 6 years old and crappy, it's the user that fails, not the computer
.
Ah, should you want to buy a serious sound card because you want to record audio to your PC or hook it up to a quality stereo set; I really like my ESI Juli@. Sounds better than most CD players
.