I have

Intel Core 2 Extreme QX9650 @ 3.00GHz
Dual 512MB GDDR5 ATI Radeon HD 4870 with CrossFire
2GB DDR3 SDRAM at 1600MHz
MS Windows Vista Home Premium 32-bit SP2

I am thinking about saving my money and buying a 2GB ATI Radeon HD6970 or HD6950 for Skyrim.

Will I see a good different between the HD 4870 with CrossFire & the HD69XX? Should I upgrade?

upgrading would be beneficial, but only in speed unless skryim takes advantage of dx11 besides speed increase.

If you want to drop the cash for it, go ahead, you're not shooting yourself in the foot if you don't...

You'll be fine with Skyrim (so long as you don't plan on playing in say 2580x1800 resolution, or on a multi-monitor setup). But if you have the money to upgrade, and are planning to play more than JUST Skyrim, I certainly would.

You should, new ATI cards are great...
and I would get Windows 7 64bits and 2gb ram more if I were you.

do you think a geforce 460gtx 1gb will run skyrim well?

Yup

Step 1. Wait for Skyrim.
Step 2. Check out Skyrim on your system at highest detalisation.
Step 3. If you're not satisfied - but AMD HD6970.
[option]
Buy 2 more monitors and HD6970 (possible two of them - but only after checkout!), run with eyefinity. Feel the difference.