I know what Quark will run in. My main G3 is getting a Sonnet ZIFF G4-800 CPU upgrade. It has 1gb of RAM (max) and does run just fine. The reason we are upgrading is because a majority of our customers are beginning to supply us with documents that we can not access, and in the current stressed printing market, we can't AFFORD NOT TO. I am also forced to work on PCs with Publisher 2003 and am constantly being given MS Word documents that I have to convert to PDF and print separations from (try that one).
I also am an InDesign user and have switched from Quark as my main tool about a year ago, yet InDesign still has a few issue with converting documents that have come back to bite us. For example I just recently opened a simple 2/color business card in Indesign for a client that we printed last year using Quark. The colors used in Quark were black, PMS 186 (PANTONE 186 CVU), and white, when open in InDesign the white I manually change to "Paper" the black was OK but the PMS 186 did not Translate because now InDesign handles coated and uncoated PMS colors with a single designation. either "U" or "C", not "CVU" and "CVC". Because of this, the logo on the business card when opened in InDesign converted to black instead of PMS, and got printed that way. We ate that one!!
We are members of Adobe's ASN (Adobe Solutions Network) which gives us all the software in the latest versions in both PC and MAC format. Meaning we gat Creative Suite (Photoshop CS, Illustrator CS, InDesign CS, Acrobat Pro, and Adobe Go Live) for both platforms and all the upgrade releases as they come out, and without OS X the MAC versions are useless.
So after that long winded explination, that's why we are upgrading.