These types of questions have been asked and answered again and again here, so you can search for more info on the topic.
Neither the RAM or Video card will help you with FCP or rendering. The 2 biggest factors are Processors and Hard drives; the faster those are, the faster you can be. 7200 FW HDs are really the slowest of the slow for performance, you'd to a lot better with a faster array. eSata, SATA, SAS, fiber, etc.
If you had another computer to could off load the rendering to that one so you could keep working? But RAM and Video would do much for speeding up FCP. Sorry.
Saying "HD" is like saying I drive a "CAR" there are many versions of HD and each one requires different specs.
You can edit okay with DVCProHD or ProRes if you have at least FW 800 drives (enclosures that have 2 drives striped together are even better) as long as you are talking about simple narrative editing, Not much layering and effects. Faster drives like a RAID system will also help on somethings.
It might be worth it to max out your RAM if you find a good deal on it (though I'm fairly confident that your system only supports 8GB total... only the G5 Quad supported 16GB). I doubt that's your limiting factor though.
Like others said, make sure you have lots of fast hard drive storage (not Firewire). Hard drives are cheap these days, and you can always move them over to your next system when you upgrade. I use a Sonnet G5 Jive in my trusty G5 to mount a total of five drives internally (one for boot and four in a RAID configuration with a Sonnet RAID card). I think B&H still carries this setup and it works flawlessly.
Don't spend money on a video card or anything else for the system. The G5s are still great, but rather obsolesced... the processors will be the unchangeable bottleneck.
I think that with that processor, adding more RAM wouldn't be of much help.
BUT 4.5 GB RAM is a HORRIBLE number. That means an uneven distribution of the RAM chips that can cause problems.
A better Video-card will be helpful if you would be running an Intel MAC (Color, fxplugs rendering, etc).
On a PPC won't help on FC processes, but in general should help for displaying.
However I'm not sure that you will find any faster Video card for that computer.