512 probably not necessary. The Matrox quads came with only 32MB to be split over 4 monitors. In my W98 rig (4 monitor), the video cards had only 4MB RAM, and they worked perfectly. I've recently had video cards with 16MB, no problem... currently using Quadro NVS with 32MB per port.
The quad cards market are for towers that do not have an available slot for an additional video card. I have always had great success running dual multi-head cards. It is merely a matter of available resources.
be careful when you shop for your cheapie $25~$50 cards on eBay. 1. make sure you know what kind of slot your machine has. Order the card that fits the slot. http://en.wikipedia.org/wiki/Video_card http://en.wikipedia.org/wiki/PCIE http://en.wikipedia.org/wiki/PCI-X 2. get the card with higher video memory. A lot of people are ditching their old card because the low video memory can not do what they want. 3. some dual cards come with a Y-cable. This type of card has only one socket. You will need a special Y-cable to connect to 2 monitors. Make sure the money you pay includes this special cable. If you have to buy a new cable, I can tell you it might cost more than the card.
Does not matter for trading. ANY card which is compatible with WinXP will have a mimumum 16MB per port, and that is plenty... overkill even.
Of course you want your card purchase to come with the necessary cable. But if not, you can count on getting a cable for $8-$9, or less (including shipping), on eBay.
This is why two video card setups... Are not often used by system builders. Driver conflicts. Instability. Sophisticated users can tinker with these setups... And get them to work OK... But no manufacturer or re-seller recommends this. For a trading business... Reliable, stable, redundant off-the-shelf hardware is paramount... And it's never about saving a few dollars.
You are absolutely correct, stability is paramount. Driver conflict can be addressed by using identical cards. On my Dell XPS 710, a second Nvidia 8600/8800 256 dual DVI came as an option.
Correct about the potential driver conflict... And I don't know what system builders "recommend", but here are the FACTS: 1. If you don't know what you're doing, you CAN thoroughly hose* your system setup with 2 or more video cards. 2. All of the potential driver conflicts are EASILY avoided with a little knowledge. 3. Running 2 video cards is not uncommon these days. 4. I'd run 3 and 4 video cards in various systems for 8 years with no problem. (W98, W2K, and XP rigs). Currently running two dualheads. * I've done it to myself and had to reinstall the OS to get rid of the troubles.