bdeditch,
You've got a choice to make, but you really can't go wrong either way if you follow through and learn correct techniques. If you can find a local WWer to help you out in person with what others are telling you here, it might be easier and quicker.
1=2E Getting a new TS. The Ryobi is probably the most economical yet decent TS out there, and (but?) it is very innovative, and if you have Luddite tendencies you may not take to it. DAGS using my name and Ryobi and you'll read lots of threads discussing its pros and cons. To find out just how innovative the Ryobi is, check out the web site devoted to it by its affectionadoes:
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Rigid models look pretty solid to me--they are a step up from the Ryobi in regards to solidness, but that comes at a cost too. They've got some proponents here.
You probably won't go wrong with either saw, provided that you learn to use it well.
2=2E Circular saw with cutting platform (either foam panel on the ground or frame on sawhorses) and a cutting guide ("sawboard"--alexy gave you a nice link there). Don't discount this suggestion, which predominates your thread. Many WWers with nice cabinet saws still use this method to cut large sheets of plywood--it's the easier way unless you have very large extensions with a sliding table.
Foam w/ guide is probably the easiest way to go, but if you opt for a frame on sawhorses w/ a guide I can suggest a very easy alternative to making the frame: go out and get a trashed panel-and-frame door ($5 at most thrift stores, free at most dumps), take the panels out and you'll have a great frame w/ straight-grained lumber. Beats all the 2x4 stuff in quality and in ease of acquisition.
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No one has mentioned it yet, but if you want *really* accurate edges then you'll have to get *really* accurate tools. It's hard to get a finished edge w/ inexpensive TSs for large sheets of plywood. It can be done, but good technique and a very good blade are the minimum requirements. The circular saw method can also be made to work, with a good guide, good technique, and a very good blade, but it may be easier to cut a little proud, then clean up the cut with a router. I know that means yet one more tool for you perhaps, but that's the way many (most) WWers attack the same problem you face.
My 2=A2 H