I think I've weighed in before on this topic, but I'll go ahead and toss another 2 cents in.
First to agree with a couple of Cialtras' points -
-A poor machine can contribute to a poor scan. If you are going to put valuable comics on the scanner or do a good number of scans, it is absolutely worth it to buy a good scanner. The better your raw image is, the better your end result will be. All in one units, old machines, etc. can produce scans that can't much be helped in photoshop. To get a good end result, it really helps to take a good picture in the first place. I'd agree that Epson makes good scanners (I use the v300 as one of my main machines) especially for sharpness of line. The scanner software works well across operating systems and the price is right ($90). My one note here, though, is that you have to be careful about prescan settings to get the colors right. Another scanner I love is the Plustek 3600. Perfect colors every single time, truly remarkable. However, I'm wary about recommending the scanner because of problems many have had with bulb life, longevity, and poor customer service.
-Cialtras is absolutely right about blur/noise filters and even descreen options. All of these can lead to blurry text (not that all golden age comics were printed with sharp text in the first place). In general, the scanners of old paper are pretty good about this. We like to see paper grain, the individual dots, and sharp line work. Some scanners use a little smart sharpen in their edit to help with this.
As for dpi and page width, there is more than one way to skin a cat, but I do prefer at least a 1200 pixel width as something of a minimum. Jim is right in that, as far as the raw scan goes (and to some extent the end product), scanning in higher dpi means sharper line work, better reproduction of paper grain and dot patterns, and clearer text. I've heard the Marvel scan guide goes with 600 dpi, and many archival projects use this as a baseline.
I personally scan at 400dpi (up from 300dpi which I started at) which I find to be a nice compromise between speed and quality and file size. I scan in tif, edit in tif, and lastly share a sized-down jpeg (this way I can share larger versions of my scans in the future if it is ever warranted). There are two options here for sizing down, the most common method being to reduce all the images to a standard with 1280, 1680, etc. and reduce jpeg quality so that each page is in a target range (I personally try and go for about 1MB a page). A second option, one which I've been toying with more, is to keep the original page width and save at a lower jpeg quality. Peeps who gather images from Heritage auctions might have noticed this approach in that their images of paintings can be a few thousand pixels wide yet weigh in around a MB in file size. If a 2500 pixel width page is saved at 3jpeg quality looks exactly like a 1280 width image in the viewer, why not share the page at original width (with no resampling on the width reduction)? For pulps in particular, I like this choice because it allows for the option for viewing with large text for the eyesight impaired, but I also like the choice of cutting out a single panel in a comic and having it fill the entire screen. Just something to think about that deviates from most scan guides you will find out there. I'm not sure how comic scanners arrived at a mostly uniform practice of resizing to 1280, but there are other options out there that will not necessarily increase file size.
(and really obsessiveness about file size is short-sighted. In the short time I've been scanning the accepted file size has grown steadily and it will continue to grow).
Anyways, I'm rambling and have lost any focus I might have had when I started to type, so I'll stop!
P.S. Thanks to all of you scanners who scan and share, no matter how you choose to do it - the choice is yours along with the effort.
