Osbauro,
I think there are 2 or 3 things that are essential to a computer based home recording set-up. You've got plenty of computer - bout the same as what I've got, and it works just fine for me.
Your video card should present no problems. You could go with a dual monitor card thing, but your video card won't determine the sound quality you'd get from your recording.
Some peripherals and cards can interfere with other cards, though. When I first started out, my sound card and my recording sound card were using the same IRQ numbers, so I had to do some tweaking, but the problem was easily solved.
You are going to need a sound card/sound interface dedicated to recording audio. The choices out there are endless, and you can go with a PCI based card and break-out box, USB interface, firewire, etc. A good place to start in my opinion is the M-Audio stuff. I've used an M-audio delta44 for years and it's been perfect for me. The delta44 that I've got has 4 1/4" line inputs and 4 1/4" line outputs. This means you can route like this:
Guitar/keyboard/etc -> card (delta44) -> computer
or
Mic -> mic pre (either mixer or outboard mic pre) -> card -> computer
Now, you have to determine how many different inputs you want to use at one time. If you are just recording yourself, 2 or 4 is good - I just record myself and i never have used more than 3 at once (I would go with more than just 1, though, for stereo micing guitars, etc). The delta cards are cool, but if you're looking for something a bit higher quality and more inputs I'd go with something like the E-Mu 1820M or
the M-audio delta1010 or delta1010lt. The E-Mu card is cool cause gives you 2 mic pre-amp inputs (so you wouldn't have to buy a mixer or mic pre's) and 6 line inputs, with 8 line outputs. You also have an ADAT input, so you can buy something like a behringer ADA8000 or M-Audio octane and get 8 additional separate channels via ADAT, bring your total to 16 separate channels. The card is a bit pricier, but better quality and more expandibility.
The next thing you need after the card is a mic pre (or 2 or 8). If you go with something like the E-Mu card you won't need it, but if you go with a card with only line inputs you will. You can either buy outboard pre's, which are just a pre and nothing else (like the M-Audio DMP-3 - it's 2 good pre's for $150). You could also go with a mixer, which is a good bet since you can use it for multiple purposes. I have an old Behringer mixer, and I recently upgraded to a DMP3 for better mic pre's, and there's quite a difference. If I were you and got a strictly line-in card, i'd probably go with a yamaha mg-series mixer for mic pre's. Get whichever model gives you enough mic pre inputs.
Now that you've got 2/3 of your signal chain covered, you're gonna need some mic's. This is a whole new world of chioces, but the Studio Projects mic's and the MXL mic's are popular. I would get an sm57 as well. Again, your needs determine which mic's you would want. I would go with a good large diaphragm condenser for vocals and acoustic instruments. If you want to get fancier, I've had good luck with a pair of small diaphragm condensers for acoustic guitar. Drums are another story, with more and different kinds of mic's needed.
Finally, you have to hear your mixes, so you'll need some headphones for tracking, and some good monitors for mixing/mastering. Again, lots of choices, but Event, M-Audio, Behringer, and Diamond make some good budget monitors.
Granted all the gear I've talked about is budget gear. You can spend waaaaay more money and get better quality.
Hope this helps, and hopefully I didn't just write a bunch of stuff you already know.
