Upgrading mobo and cpu from q8300

Hello once again, peoples....I've been around the grapevine and I've been considering upgrading my mobo and cpu. Here's my system:

mobo: ASUS P5G43T-M PRO
P5G43T-M PRO
CPU: Intel C2Q Q8300
RAM: 6 GB DDR3 RAM (PC3-10700)
GFX: HIS ati radeon 5850
HD: 600 GB
PSU: OCZ ModxStream Pro 500W

Since LGA 775 is now a dead socket, I've been considering switching to AMD and getting a Phenom II X6 1055T with this asus motherboard:

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

I'm trying to keep a reasonable budget under $500. I'm also wondering if switching to amd will grant me better longevity...or should I save up for Intel- based systems? Any input is welcome! Thanks!