Of course its reasonable if you want AM3 or AM3+
Like MIND mentioned you'll need to upgrade your memory, I used kingston mostly or mushkin because both seem to get perfectly fine with AMD builds.
Newegg.com - Kingston HyperX 2GB 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model KHX1600C9AD3/2G
This model I have but I have a 4GB 2 sticks, they are quite good for gaming and everyday use, but pathetic at times when trying to do extreme OC's.
The video card for picking....
Newegg.com - GIGABYTE GV-N550D5-1GI GeForce GTX 550 Ti (Fermi) 1GB 192-bit GDDR5 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card
About the higher end phenoms ii and motherboard if you want something abit more stronger you have quite a few options:
1045T hexa-core
Newegg.com - AMD Phenom II X6 1045T Thuban 2.7GHz Socket AM3 95W Six-Core Desktop Processor HDT45TWFGRBOX
or
Newegg.com - Computer Hardware, CPUs / Processors, Processors - Desktops, Phenom II X4
That Phenom II X4 965 will do you great justice and is very good for medium high end gaming
Another mobo choice is this:
Newegg.com - GIGABYTE GA-970A-D3 AM3+ AMD 970 SATA 6Gb/s USB 3.0 ATX AMD Motherboard
Is a video card upgrade necessary? I'm not going for the absolute extreme, mostly looking for support for mid-high range graphic demanding games, which i THOUGHT could be done with my current ATI Radeon HD5700. If this is the case, can we conclude the following will have me up and running with improved fps?
Motherboard: Newegg.com - GIGABYTE GA-970A-D3 AM3+ AMD 970 SATA 6Gb/s USB 3.0 ATX AMD Motherboard
CPU: Newegg.com - AMD Phenom II X4 965 Black Edition Deneb 3.4GHz Socket AM3 125W Quad-Core Processor HDZ965FBGMBOX
RAM: Newegg.com - Kingston HyperX 2GB 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model KHX1600C9AD3/2G
Also, should I have this RAM in all 4 memory slots?