I also bought the Matrox G200 Multi-Monitor (G2+/QUAD-PL/TVN) a little over a year ago and really like it. I have had great luck with it. Also, with it I have had no problem using a AGP video card at the same time for a fifth monitor. I have heard of people with some other Quad cards having trouble with this. Also I have had no problems when I close a program with it opening back up on the #1 (main) monitor, opens up exactly where it was when it closed. I think it is a really hard card to beat.
I have an Appian Rushmore 64 MB DDR (Latest serie) for sale (It was a gift, already bought one myself) Full waranty PM me if interested.
Actually the Matrox G450MMS Quad card is better and newer than the G200MMS Quad card. The G200 card has 32MB total memory while the G450 has 32MB memory per monitor or 128MB total on the card. The G450 is digital. I don't think the G200 is. The Matrox website will have that info.
Be sure you really want a quad instead of a couple duals. Most decent mainboards have a ton of PCI slots - if you have a box you(or your neighborhood techie) built, chances are you have the slots. Never ran any quad cards, so I cannot comment on which one is better.
The G200 MMS series is both digital and analog. I have four of these and they have work perfectly. For trading, 8MB of memory per monitor is more than enough for my needs.