Was that 2EA 4FT X 8FT sheets of 3/8" plate, OR 4" x 8FT strap? If 4" strap, go buy a chop saw as you will burn up a circular saw trying to do much cutting on 3/8" plate.
If it is 4' X 8' plate, a torch or a plas is your best bet. Another option is possibly to get the major cuts done by the people providing the steel. A 4' X 8' plate of 3/8" is pretty difficult and sometimes dangerous to move around without the right equipment to do so properly. If you can get the major shapes(squares and rectangles) sheared by the supplier, you will probably be time and money ahead as a few cuts shouldn't cost that much from the people with the right equipment to do it properly. If your plans are to turn that 4' X 8' into a lot of little parts, you should probably bite the bullet and get a torch as you will spend a long long time trying to cut them with anything else.
I used to be a welder and am really surprised I have gone this long without getting at least a torch. I am about to now as I have some frame work to do on some trucks. Before I would use a handheld circular saw, I would recomend a large jigsaw or sawzall with bi-metal blades. I built a fork bucket for my tractor last summer with a chopsaw and a hand held jigsaw with bi-metal blades to cut the triangular gussets that support the 11 forks. That was 6 cuts about 12" long in 1/4" strap(cut 6" X 12" strap corner to corner) that took about 10 minutes each to complete. This actually worked well as it gave me a chance to stay within the duty cycle limits of my welder. If you use a sawzall/jigsaw, you want one with a blade holder that will hold the entire blade width. In thin stock, the blade wears in a very narrow area. With the right type blade holder, you can break off the base of the blade in a vise and re-insert it into the saw to cut with a new section of teeth. When cutting thin stock, I can usually break off a standard sawzall blade about 4-5 times and use nearly all the teeth on the blade before I need a new blade.
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