With your current motherboard you can run the new video card, all you would need is a bigger power supply to make your system handle it. Seeing as how you already have a lga775, I would get the video card/psu first (to get you up and running) then upgrade the mobo (keep the stock cpu/hs) and when more money hits upgrade the cpu/heatsink. you don't necessarily need to upgrade your motherboard/cpu, but it is a big bottleneck, and any improvement here will help alot.
Just my 2¢
Also, I just did a little research, and you have 244mm (24.4cm, or 9.6") between your I/O sheild on the back and your hard-drive bay, so the card might be a little bit of a squeeze (they are about 23cm or 9", so that gives you about 1/2 of an inch (12.5mm) of clearance for the power connector on the back of the card. It should fit.
Page for the motherboard on ECS site:
ECS Web Site
This heatsink should do you pretty good
Newegg.com - ARCTIC COOLING Freezer 7 Pro 92mm CPU Cooler - CPU Fans & Heatsinks
paired with this cpu maybe too
Newegg.com - Intel Pentium E6300 Wolfdale 2.8GHz 2MB L2 Cache LGA 775 65W Dual-Core Processor - Processors - Desktops
board however, not my specality, as I am an AMD guy aswell...