SSD all the way, baby!
Go with the SSD!

I am in the same boat as you. I have a Mac Pro (8-core from Early 2008). I have 8GB of RAM. More than enough regular hard drive space.
I recently debated getting another 8GB of RAM to speed stuff up (16GB total) or get an SSD for the boot drive. I opted for the SSD boot drive. Why? Because my computer barely uses the 8GB of RAM that I already have!
I, like you, run Final Cut Studio, Photoshop CS5, and many more all at once for editing. Like I said, the 8GB of RAM is barely what is slowing things down. It's the hard drive.
Everything is soooo much faster now. Quick stats:
Before (500GB Seagate Boot Drive ST3500630AS)
Sequential
Uncached Write 28.45 MB/sec [4K blocks]
Uncached Write 47.21 MB/sec [256K blocks]
Uncached Read 11.40 MB/sec [4K blocks]
Uncached Read 46.28 MB/sec [256K blocks]
Random
Uncached Write 1.76 MB/sec [4K blocks]
Uncached Write 29.35 MB/sec [256K blocks]
Uncached Read 0.59 MB/sec [4K blocks]
Uncached Read 21.00 MB/sec [256K blocks]
After (256GB Crucial SSD CT256M225)
Sequential
Uncached Write 218.13 MB/sec [4K blocks]
Uncached Write 154.59 MB/sec [256K blocks]
Uncached Read 43.33 MB/sec [4K blocks]
Uncached Read 241.33 MB/sec [256K blocks]
Random
Uncached Write 12.16 MB/sec [4K blocks]
Uncached Write 202.82 MB/sec [256K blocks]
Uncached Read 24.55 MB/sec [4K blocks]
Uncached Read 171.03 MB/sec [256K blocks]