Help me upgrade my computer!

[Hawkwings]

So, my computer needs an upgrade, especially if I'm going to go buy Oblivion and play it without it looking like a slideshow.

My computer currently:
No idea on motherboard model
P4 processor running at 1.7 ghz
Geforce 5600 XT graphics card, 128 mb
512 MB RAM, in 4x128 mb sticks

and, if for some reason my OS is important, I'm running Win2000 SP4

Now, I'm looking for a new motherboard/CPU combo first and foremost. After that is more RAM, and then a better graphics card.

My requirements:
I need a computer that is rock-solid. My computer hasn't given me any trouble in the past 3 years and I'd like to keep it that way. When there's a choice between performance and stability, with comparable price, I choose stability. I wich to avoide situations like "I found this incredibly awesome item, now all I have to do it click it to pick it up..." followed by the computer shorting out.

I'll have around $300-$500 to spend, and I'll be buying this in about a month.

Suggestions?

[Uraniun235]

A really quick skim of Newegg shows that you could probably grab:

Athlon 64 3200+
ASUS A8N5X motherboard
Geforce 6800GS vid card
1 GB (2x512MB) RAM

...for ~$425.

And in case your current power supply isn't beefy enough to handle that, Antec offers a decent 400W power supply for ~$55.

[phongn]

Uranium's solution is good, but the fan on the A8N5X is pretty lousy so you might want to consider trying to get a replacement for it.