Upgrade of my Dell Precision 490 from 32-bit to 64-bit?

After reading other threads here about Dell 490 upgrades, I am looking at my existing Dell Precision 490 with 32-bit XP, Intel Xeon 2.67, 2GB RAM...with only a total of 240GB HDD space...and given my need to upgrade from my Avid MC 5 to edit more HD video (upgrading to Avid MC6, requires 64-bit, Windows 7) wondering what it would take, and more importantly whether worth it to upgrade my 490 to 64-bit and also increase HDD space to maybe 1-1.5 TB, , maybe add a USB 3.0.

To buy a whole new system looks the better part of $3K and I'd prefer to get a couple more years out of this one.

Approximate Purchase Date: ASAP
Budget Range: $500
System Usage from Most to Least Important: Video Editing, Avid Media Composer
Are you buying a monitor: No (Have dual Dell 1908 WFP's)
Do you need to buy OS: Yes, have XP, assume I should go to W7 given Avid
Location: Albuquerque, NM
Overclocking: Unknown subject
SLI or Crossfire: Unknown Subject
Why Are You Upgrading: New Avid MC6 requires 64-bit Windows 7, I am also seeking faster speed and utilization of the 490's extra RAM capability (6 empty DIMMs).

Thanks,
Bob