BuildCraft gates can be configured to throw out redstone signals when items pass through them, but it'd be difficult to use them to determine type (especially since a fast flow of items, let alone STACKs of items passing at a time, may be uncountable).
I made a system similar to what you're thinking where each barrel had a gold OR gate above it, set to transmit a redstone signal when the barrel underneath had no more room for the specified resource. Each bank of sixteen barrels connected to a redpower bundled cable, and each bundled cable connected to a computer.
The turtle floated around collecting bee combs, dragging them to the centrifuge, combining the dusts this produced, then smelting/compressing the results. It used rednet to communicate with the computer to determine which barrels were full (and hence could be avoided - you'd be amazed how fast an alveary filled a barrel of lapis blocks), but it couldn't get actual item counts short of going over to a barrel and trying to pull a stack out.
Though bear in mind that ComputerCraft's rednet is entirely broken if using it with turtles under MineCraft 1.4.7. It led to me giving up on the world, because once I'd finished my system, the MineCraft server averaged about three hours of uptime before stalling.
If memory serves, you may have access to an "inventory turtle". I've not tried it myself, but I believe it's shtick is reading the contents of nearby containers.