what kind of sound card should i get for my dell 4600c?
Well we'd need to know what kind of slot it has and how many are free. It also depends on what kind of audio speakers you have / plan to use : if you only have 2 speakers, no need for a 5.1 board, etc...
I couldn't find precisely the specifications of this machine ... please run Everest, gerenate a plain text report of the hardware related pages (using the report wizard), and copy/paste it here, so we can see what you have.
If it's the machine I'm thinking of it doesn't have any expansion slots, at all. So you'll be needing an external sound card of some kind, perhaps a USB one like this: http://www.creative.com/products/product.asp?category=1&subcategory=206&product=10702
It won't be especially brilliant, but it's cheap.
Do you plan on upgrading to Windows Vista anytime soon. If so, then you should NOT spend a lot of money on a fancy sound card. Vista emulates the sound card, so every card with 5.1 jacks will sound the same.