Digital BW, The Print

List,
I am interested in comments on process and equipment.
I currently am making pleasing prints with an Epson 3000 and Piezotones, shooting with 
Canon D10 in raw. Converting to b&w 16bit. No display calibration or purchased profiles. 
Using QTR. All trial and error I suppose. Lots of test strips and the best 
softproofing I can manage. Working on Mac, first gen mid tier G5 with Apple 21" display. I 
would like to print the images much larger! I will outsource the printing. But, my files are 
not suitable right?
When opening the raw image I use the 'raw' preview environment in PS and turn down the 
saturation completely, adjust the various image sliders till I get a decent histogram. I open 
as rgb, 16 bit, 300 ppi and at 6144 x 4996 (is this interpolating the image larger than its 
native res?). After converting to grayscale I end up with a 48mb, 16bit image. After tweaks 
and very minor cropping I have roughly a 20 inch image at 300ppi. How large can this go?
If I want significantly larger prints am I better off shooting large format (4x5) and drum 
scanning the negs? I would not have access to or a budget to rent some mega-mega 
digital camera.

Thanks,

Jamie