New Motherboard/Chipset - questions for PC builders

I'm looking to upgrade the core of my system without having to replace a lot of the peripherals - ideally, I'd like to buy a new motherboard, a new chip, and keep everything else, including the case.

Here's what' I'm replacing (purchased ~4 years ago)

Motherboard: Gigabyte GA-MA790XT-UD4P Socket AM3 AMD 790X + SB750 Chipset Dual-Channel DDR3 Audio Gigabit Lan IEEE1394 2x PCI-Express 2.0 Graphic Slots ATX

Chip: AMD Phenom II X3 710 (95W) Triple-Core Socket AM3, 2.6 GHz, 7.5 MB Cache (512KBX3 L2 + 6MB L3 Cache), 2000 MHz HT, 45 nm

When I purchased them, together they were $270 Cdn. That's around what I'm looking to spend again.

Currently in my system is some DD3 Ram, a single graphics card (1GB memory), 2 SATA hard drives, and an old SoundBlaster X-Fi card that I largely hang onto for the front-panel input/output options. My case is running a 500W power supply.

What's the best bang for my buck these days? I don't do very much HEAVY gaming at the moment, and the biggest drag on my system seems to be Kerbal Space Program. GW2, maybe, too, but that runs pretty smoothly most of the time. What questions should I be asking? Will I need a new case with the new motherboard (alternatively, how can I ensure that I won't need a new case?)