LTG,
I have
a tm-d1000 and a fd4 and use both with my computer, so I will give you the basic comparison.
The tmd1000 is a wonderful tool for both recording and sound reinforcement, and it offers some great effects. I use mine with a Mixtreme TDIF card and with one cable you get 8 in and 8 out of your computer. The mixer is quiet and has a very good feel, once you get used to it. It is much harder to use right out of the box then the fostex, but after a little time it all comes together beatifully. I love the ability to both record and playback with one cable.
The FD4 is a wonderful tool for recording, although the soung isn't as good as the tascam's when going direct to computer. It also doesn't have the ability to record 8 tracks at a time. However, I love using my tmd1000 and fd4 together when recording anywhere but home, and I must say I have been quite impressed with the results so far. I use a zip 250 drive when recording gigs and whaterev else, and when I use the tmd1000 as a submixer and effects unit, I have gotten great results.
The prices of both of these units is CHEAP right now, although the fd4 doesn't come with a drive. At musician's friend the tmd1000 is $499 and
the fd4 is only $299, so the total cost is about what an fd8 is, I think. I already had my zip drive when I bought both of my units, so I got lucky.
Overall it depends on what you want to do. If you are going to record strictly at home, get the tascam. If you want to record somewhere else and do not want to carry your computer (tried it, it sucks!)get the fostex, although if you go the route I did you can get the features of both for CHEAP.
If you have any more questions, post 'em and I'll see if I can help.
Madadam