RPC:
It's the Matrox Millennium G400 that I saw that had limited resolution for dual monitors. I've not seen the G450. The issue came up when I created a couple RealTick trading pages for a friend, and they could not be displayed at the proper resolution with the G400. The pages were created for dual 19" monitors at a resolution of 1280x1024 each (2560x1024 total desktop). The G400 could not display this resolution. I reworked the pages on his computer at the smallest resolution available, but the result was not great. Kind of like reading the large print edition. Page elements took up way more monitor space than they needed to.
Maybe the G450 has greater resolution options, but before I bought it, I'd call Matrox to be sure it supports 2560x1024 (for two 19" monitors) and provides that "screen area" pixel size as a setup option in Windows display settings.
Regarding the networked computer question, I'm not familiar with the LinkSys router/switch, but to add a 3rd computer, I suppose you can add a 4-port hub with crossover cable between the router and hub, then connect all three PCs to the hub, or leave one PC on the router/switch and two on the hub. As to the dissimilar operating systems, I haven't tried sharing files between a Win98 and Win2K computer, but I doubt Microsoft would make them incompatible. If you are not sharing files, the dissimilar operating systems are not an issue. If a peer-to-peer LAN (no server), although all PCs are on the same LAN, they operate independently.