If you want to scan 8x10" negs and transparencies, the best deal out there that I found is to get an older Agfa Duoscan (the HD is the best model, but any will do). You'll need a SCSI card, if you don't have one already, and the scanner should be under $150 depending on model and condition. I recommend scanning negs and transparencies directly on the glass plate with a sheet of anti-newton glass over it for sharpest results.

The Agfa scanner interface is the best OEM interface I've used on any scanner. You can do quite a lot of tweaking before you get to Photoshop with decent results.

Of course it may turn out to be older than your current scanner, but these were originally pretty high end pre-press machines. Something newer will be much better, but for 8x10" it's tough to find anything new under $1000.