If you really want to throw money at the machine, get a min 128 GB SSD drive for OS and applications and install a second drive like a 2 TB Western Digital Black or 2 TB Hitachi 7K for user data and scratch while rendering.
OR, as suggested, setup a RAID. Get a Firmtek PCI controller card that has port multiplication and connect 5 drives to create a RAID 5 array.
Add to the drives a used Geforce 6800 Ultra or Radeon X800XT to give your desktop rendering the maximum OpenGl performance possible.
These are all possibilities, but after spending the required $600-$1000 (or more), you still have the limit of 4 GB RAM and the 1.8 GHz limit, both of which are low and slow for late model software requirements.
And, all that you will gain are a few seconds off the several hour video rendering times WITHOUT full HD support.
I LOVE PPC Macs. THAT should be obvious to everyone. You realize that and I appreciate it.
BUT, as you also realize, you can't realy go faster, only optimize performance .... and that costs money.
Do it fo fun and hobby and have the money, GREAT.
I'm all for it and have several "black hole" projects.
For practicality, less than $1000 builds a blazing fast 3.5 GHz i7, 16 GB 1600MHz RAM, SSD, Radeon HD 6870 equipped machine that will render HD in minutes rather than hours, hours rather than days.....
$600 buys a faster, more capable Mac Mini.....