Well, you will want to upgrade to el capitan - here's a link to get started: http://forums.macrumors.com/threads/2006-2007-mac-pro-1-1-2-1-and-os-x-el-capitan.1890435/

As for dumping in money, the first thing you'll want is a better graphics card. I'm not sure if it's really worth the money to get a really good one for such an old rig. If it were mine, I had the cash, and I really liked it for aesthetics or nostalgia, perhaps this: http://www.amazon.com/Sapphire-Edition-PCI-Express-Graphics-11196-15-40G/dp/B00BW0XDU6/ref=cm_cr_pr_product_top?ie=UTF8

Before doing anything, I'd upgrade the OS and play with it doing editing, gaming etc. and see where there are issues that bother you, if there are any. After that, see whether or not it's worth it to you to spend any cash on it.

Best of luck!