D70 largest image is 3008 x 2000 pixels = 6016000 pixels
350D largest image is 3456 x 2304 pixels = 7962624 pixels
At 72dpi (dots per inch):
D70 will generate an image of 3008/72 x 2000/72 = 41.8 x 27.8 inches
350D will generate an image of 3456/72 x 2304/72 = 48.0 x 32.0 inches
D70 will generate an image of 3008/300 x 2000/300 = 10.0 x 6.7 inches
350D will generate an image of 3456/300 x 2304/300 = 11.5 x 7.7 inches
For reasonable print, 250dpi is enough:
D70 will generate an image of 3008/250 x 2000/250 = 12.0 x 8.0 inches
350D will generate an image of 3456/250x 2304/250 = 13.82 x 9.2 inches
Just have to take note that you set the dpi to get a desired print size. The image resolution is always the same, either the 6 megapixels or 8 megapixels.
Looking from the point of view of the printed images....
4R prints (4x6 inches):
D70 will yield a 3008/6 x 2000/4 = approx 500 dpi image
350D will yield a 3456/6 x 2304/4 = 576 dpi image
S8R (8x12 inches):
D70 will yield a 3008/12 x 2000/8 = approx 250 dpi image
350D will yield a 3456/12 x 2304/8 = 288 dpi image
Hence, if you don't crop and use full image to print, you get slightly better resolution with the 350D. Expected, since its a 8 megapixel versus a 6 megapixel camera. Also, if you shoot at ISO1600 on the 350D, and downsize it to 6 megapixels, you would have reduced some noise too.
Of course, your choice of which system should not be based on image size alone.... other factors like lens, accessories and features come in. The sensor resolution will always increase as time goes by... ;p