There's no point building a computer which will work great with Starcraft 2 but then finding out that some new game you want to play doesn't work very well at all with dual cores. You'll only end up kicking yourself for doing that.
Do not use that old PSU! Whilst it may be okay in your current computer, it's 5 years old and in power supplies, capacitors wear out over time, meaning that it will no longer be able to produce the 450W which it says it should. Anyhow, the MINIMUM for a GTX 460 is a 450W PSU, so you want to add a little on there for safety. I'd get a 650W or higher if I were you.
Also, when operating systems are installed they are essentially paired with the system they are installed on. Moving your hard drive to a new computer (or copying everything on it to a new hard drive IN a new computer) will not allow you to boot from Windows XP. What I suggest you do is firstly build your new PC and install Windows 7, then put your old hard drive in alongside your new one and copy all the files you need from there onto your new drive. You can then use the old drive as a backup, as extra storage, or you can remove it and use it elsewhere (or get rid of it
)
My recommendations:
CPU:
Phenom II X4 955 - $159.99
Very fast quad core processor.
Motherboard & OS:
ASUS M4A87TD & Windows 7 Home Premium x64 - $189.98
Good board which supports SLI for the future.
RAM:
Mushkin Enhance Sliverline - $97.99
Decent speed, 4GB is enough for now.
Graphics Card:
Zotac GTX 460 1GB & Free Just Cause 2 - $229.99
Very good card, is enough for now especially considering you're only playing at a reasonably low resolution. Just Cause 2 is great fun too
Hard Drive:
Samsung Spinpoint F3 1TB - $74.99
Fast, spacious drive.
Optical Drive:
Lite-On 24x DVD RW - $18.99
Cheap, does the job. Don't need to spend more money on this.
Power Supply:
Corsair 650TX - $89.99
Enough power and connections for your system, won't have to worry about anything with this. Corsair is one of the most reliable companies for PSUs.
Total: $861.92 before rebates
EDIT: I hadn't seen that page on the Gigabyte card. On the Palit site and the Nvidia site it says you need a 450W PSU.