I am guessing that you have a PowerMac G4 AGP from the specs you listed. According to Apple specs it has four PC-100 SDRAM modules - that RAM is hard to come by so it might cost a fair bit to upgrade to 1GB unless you buy second hand (be wary of faulty RAM - always request a memory test!).
Tiger can be had from eBay for a reasonable price - make sure that you buy the full retail packaged one and not a gray system disk - these will not work with your machine!
The Tiger retail disc is black with a big silver X on it. As said before, you don't want the grey discs, they won't work on your machine. Tiger right now on eBay U.S. looks like the cheapest buy it now is around $60, but there are auctions with lower prices.
Tiger came on a single DVD or could be purchased as a 4 set of CD's.
If it's the single CPU 450 Mhz, it is the AGP model, and if it is a dual 450 Mhz CPU, it's a Gigabit Ethernet.
You can also use PC 133 SDRAM in it, it will automatically downclock to the lower speed. So when you search for RAM you might find a slightly lower price on PC 133 RAM (maybe not, but it's good to know and compare prices before buying any).
I have a G4 400 Mhz Gigabit Ethernet that is one of my favorite PPC Macs. It's surprisingly fast on Tiger and has the same FSB and cache as your Mac.
They can also actually run 2 GB of RAM. These are one of the many machines Apple rated at less maximum RAM than has been tested and proven to work, but 1 GB on Tiger is plenty.