The screws used on PC cases are either metric or American - the coarse ones are American, and I know that they're not 6/32. I'm a connoisseur of 6/32 machine screws; I find them to be quite useful for just about anything - and a 6/32 screw is much thicker than computer screws.
Are there any computer-ish stores near you? Go there and get some screws, of each thread type. Best way to size it at the hardware store - try the nuts they've got until you find one that fits perfectly. Then get screws of that size.
Just re-reading your original post here....you're trying to replace Torx screws? Where are these located? Are they used to hold the various panels, like the top panel, in place? Because if that's what it's for, there is no standard screw for that - a lot of cases I've seen use rivets. To get rid of those, I've drilled them out, and then used a 6/32 machine screw with a nut on the other side. I bought a full box of 100 1" long 6/32 screws - if I want any shorter, the Dremel is able to oblige.