There are a lot of threads about this so you'll need to do a fair bit of reading, even if you decide to get one. And if you decide to get one, then the next question is obviously, which one.
The main thing though that you'll immediately get is an improvement in CPU performance because a lot of the work the CPU handles gets offloaded to the audio interface. This also manifests as being able to use more VSTs and tracks in a song than without one. You'll also get less lag if using a midi keyboard/controller.
If you record your own samples or instruments, usually sound quality will go up, you'll get both higher frequency and greater bit depth i.e. better sound quality.
I won't write too much because nearly everything I could write is already answered in the various threads in this section.
You're going to pay 2-300 for a decent audio interface. I've used the MAudio 2496 with good results and have been fiending for a EMU 0404 usb. It seems to have the best DACs for the price.
I recently bought a Roland JDXi crossover synth that acts as a audio interface with midi over usb and exports in 16bit. If you're going to drop $300 on a soundcard, it might be worth it to drop the $500 on the jdxi and get some synth action to boot. Of course some audiophiles will tell you 16bit audio is crap and you need at least 24. If you're one of those, I'd recommend one of the first 2 interfaces I mentioned. You could probably get the 2496 for under $150 if you look around.
IMO, even a cheapo interface is better than a stock sound card, even if it's just for the sake of consistency between computers / being able to handle XLR / phantom power / gain staging abilities / other bells and whistles.
Don't quote me, but I've seen considerable thread successes with the formula, "Pros & cons of X (specific piece of gear)"
Well as it is I'm not "even on a stock audio card". I'm on a randomly bought laptop. With a lot of cool software and samples, granted, but I'm choking here. I feel my learning and experience exceeds my hardware capabillities! An arrogant thing to say for sure but so it feels. (I play the guitar pretty well but doesnt even have a MIDI interface for it!) So the question is where to use my money..?
Just for a bit of a curveball. DV 247 are currently doing the Behringer F-CONTROL AUDIO FCA202 FireWire Audio Interface for £9.90. It doesn't have Phantom power, but it does come with a light version of Ableton, and could help you dip your toe into the world of outboard without spending much money. Having the 2 firewire connections for that price is pretty impressive.
I got a Komplete Audio 6 recently too - I'm not sure if it's a faulty interface or shitty monitors (they were really, really cheap, so I wouldn't be surprised) but I can't get any sort of volume out of the main outputs, with XLR or RCA outputs. :/
I had the Focusrite Scarlett 18i20 and it was an excellent piece of kit - although you'd be wanting a smaller one for your purposes, say a 2i2 or 6i6.
I downsized to a Steinberg UR44 and I am pleased as punch with it. I would recommend it just as much as the Scarlett. It also allows you to insert FX directly onto your inputs in the interface's software, allowing me to put a compressor on my voice for when I stream games or art, for example. The Scarlett doesn't allow this.
I'll put in another vote for focusrite scarlet series. I have a 2i2 and it's perfect for my needs, namely recording one synth at a time, and works great with my AT-2020 XLR condenser mic I use with vocalists.
if you are ever going to be using Linux, which not a lot of people do, you will want the interface to be USB Class Compliant so that it just works without drivers. It's also kind of nice on Windows and Mac that way too.
Just for info, i'm using Traktor Kontrol S4's built in audio interface, have had huge problems with using it in FL even after few weeks of "Windows tuning". People say that NI's drivers to that are crap (which im not sure of) either my HDD is messed up and can't read too many tracks and plugins, as it sometimes drop outs, clicks on pops. Sound quality is great though.
I've been using the Focusrite Scarlett 2i2 and I think it's been good value for the money. There's a lot of decent interfaces available in the same price range though with slightly different specs so you can just pick the one that best suits you!
I use my interface a lot for recording my non-electronic music projects but other than recording I find that it has made the quality of playback a lot better on my laptop too. It can be a bit of a hassle to install these things and you have to make sure you have a decent usb-cable so your device won't disconnect all the time but nevertheless I think a usb-interface is definitely worth the investment as default audio drivers can be a bit blech.