New Pc or Upgrade?

Hello everyone! Let me start by saying that I am by no means an expert in the pc world, but I find myself in need of some help so I thought I'd write this post!

Also, my apologies if for some reason this is not the correct sub forum to post this question!

So, let's get to the point. These are my current pc specs:

Windows XP Professional 32bit
Asus M3A DDR2 Am2+ RX780
AMD Athlon 64 X2 Dual Core
4 GB RAM
500 GB HD
Nexus 500W Silent
Cooler Master Centurion 532 (case)

I bought it, mmh, 3 years ago maybe? perhaps 4. I haven't used it much since I traveled a lot in this period and I pretty much only used my trusty macbook
but now that I am settled again, I'd really like to have a nice up-to-date pc.

2 things

1: What do I need it for:
---I use vegas pro quite a bit and make different kind of videos, so I need a decent pc(for rendering&stuff)
---Gaming is always nice. I'd like to be able to the new games smoothly.

and that's it

2: I guess I could simply upgrade the thing, no need to go out and buy a brand new pc with all brand new part right?
I am as I said before, by no means an expert with pcs and such, though if there's a good guide, usually I can follow it (example: a while ago my trusty xbox 360 got the 3 rings of death. so i just looked up on the internet, found a guide to fix the problem and went on to take the xbox apart to remove some parts and clean the cpu, reapply the thermal paste etc)
So that lets me think that with some searching around I should be able to eventually upgrade-build a pc. What do you think? Is it very hard?(of course I have no clues to which parts I should buy...but that's why I'm here )

So to the main final question after all this writing!

How can I upgrade my pc to fit the 2011 good standards? (or should I just go and buy a new one?)
How can I make my pc QUITE! Damn this thing here makes so much noise that my ears hurt!…ok I am exaggerating but it is very annoying, especially being now used to the super silent mac. Is it possible to have a good pc that doesn't sound like a plane taking off?

Sooo yep I think that's it! Thank you very much to any of you who will bother helping a poor ignorant fellow

Ciao!

That setup is too old, forget about upgrading if you want 2011 standards. Do you need a PC now, or can you wait a couple months? The new Sandy Bridge E hexa-core CPUs will be coming out soon and I think that would serve you much better if you do a lot of rendering (depends on your budget though, MSRP is ~$500). If you must upgrade now go for a i7-2600k, or AMD's new 8-'core' bulldozer.

First off, what is your budget? If you have the money, i would build a new PC.

thanks for the answer guys!

budget max 1500$