If you're happy with your mics, then I would say the next steps are (probably in order, given my personal experience with roland preamps):
1) Preamp:
Cheap Behringer mixer (eg 602, $60US, 2 pres, or 802, 4 pres, $80US)
OR
M-Audio Audiobuddy ($80) or DMP-3 (not sure of current best price) (2 channel, nice)
OR
Studio Projects VTB-1 $129 (1 channel, tube)
2) Compressor
FMR audio RNC $175 - very nice
OR
CHeap Behringer, reputedly not nearly as good, but my composer pro works for me at the moment
If you don't have phantom power, are you using condensors? Most of the battery powered versions don't have a good rep here, so you might want to consider:
Pair of MXL 603's - fantastic for acoustic/piano/drum overheads $140
Studio Projects B1 - $80, good on vocals/acoustic/lotsa other stuff
MXL V67B - $80, similar applications to B1
A pair of Mxl 603's and a large diaphragm (B1 or V67) (or a pair of large diaphragms as well) will let you do nice stereo recordings of your acoustic, plus nice vocals.
You can upgrade the choices plenty from here, but those are good relatively cheap starters...
Steve