A little clarification on which EPROMs can actually be used...
Like Matthias said, there's two Atari - designed cart PCBs which can be used, the 4MB (2x2MB chips) and the 2Mb (4x512KB chips).
The 4MB one is designed for the 27C160 (as in the BS carts) which is astonishingly expensive as they aren't used for anything much any more, and are in correspondingly lower production (unless it's been stopped altogether and old stock is all the remains...I'm not sure on this point). Expect to pay around £27 each at Maplin (don't recall the exact price).
The 2MB one is slightly more versatile, but you are restricted to chips currently in production, which rules out the Toshiba one (27C4000D?). The 27C040 will work in this board, and the 29C040 Flash chip will if you have one of the jumper-selectable PCBs (the pinout is slightly different).
If you have an EPROM burner and a UV lamp to erase them with you could get a good deal off eBay - I got 10x27C040 for about £10 all told, so look around.
To find out exactly which chips are supported, go download the Jag Developer's Manual off Starcat's site (
http://www.atari-jaguar64.de) - the section you want is called Hardware Reference (I think...it could be in the Technical Reference). Just download it all, it's some interesting shiznit
If you want to design your own PCB that's also possible. Just don't expect it to be easy

It looked to me from the commercial cart PCBs (which differ from the EPROM ones btw) like the Jag cart port was designed with certain chip pinouts and cart layouts in mind, as all the tracks are pretty much lead straight down to the port which can't be a coincidence
Anyway, enough babble from me, go build something interesting
Stone