well for motion and photoshop work, you really could use all of these things.
the graphics card will increase the longevity and performance of your computer the most noticeably, this would probably be the first thing i would upgrade. a 9800 se would be a good investment as it's unlikely you would need more power, and it's going to be one of the last generations of AGP graphics, so it will last the life of your system.
a negative to the graphics card is it's "recycle-ability". your next machine will have a better graphics card, so this card won't have a further use, then again your G5 will be around indefinitely. let's face it, even at 1.6 ghz and a signle processor, that is one powerful computer.
the HD space is also pretty necessary especially in Motion and heavy PS'ing work. i find anything under 250GB to get pretty cramped pretty fast (i use Apple Lossless audio and store DVD's on disk, combined with multimedia work, space is a premium). the first thing i would do is max out the storage space and raid them for performance.
RAM is the only iffy one. 1.2GB of ram is alot, no matter the theoretical 1^64 GB limit or 8 GB practical limit. You will only notice a boost in performance when you're using some huge files, and multitasking at the same time. The RAM would be the last thing I'd worry about.
I'd upgrade the HD now, it can be used in your next machine.
Wait for the price on the 9800SE to drop (around time 6800 is widely available). Then upgrade to the 9800SE or equivalent.
RAM can wait until you really feel the need. Next machine's ram will most likely be of a different type.
didn't mean to imply your G5 is crap, just that you got the shaft thanks to apple's marketing. it happened to everyone who got the MDD's, and it most likely just happened to me when i bought my Rev. C powerbook. it happens to everyone. im still considering upgrading my G4 (until G5 tops 3+ or G6), so...sorry if i rained on your parade.
