I concur, a solid state drive will make a world of a difference. Video card is always great, especially if you have one already. The most economical CPU upgrade is to X5355 Xeons, which are Quad Core 2.66 GHz, and can be found at much cheaper prices than the X5365s (Quad Core 3.0 GHz) - I've grabbed a matched pair of 2.66s for $50, while I see 3.0s go for almost $150. However, if you don't feel that you need the horsepower, I'd invest in the SSD and RAM before anything else.
16GB of RAM for a 1,1 can be purchased at a reasonable price if you get it in an 8 x 2GB stick configuration. People often say that the FB-DIMMs that the older Mac Pros take are expensive. This is certainly the case if you try to get sticks larger than 2GB, but 2GB sticks are fairly reasonable in price, I've picked up a couple of sets of FB-DIMMs in the past, typically between $35-$40 for 4 x 2GB sticks. That's about $80 for 16GB of RAM - heck, that's CHEAPER than 16GB of RAM for DDR3 machines. The real problem is when you want to get MORE than 16GB. Prices go through the roof on 4GB sticks. Stay away unless you can get a good deal.
Hope this helps a bit, good luck with your upgrades.