I have used only 2 makes: PNY Technologies http://www3.pny.com/Categories/GeForceGraphics.aspx?Category_ID=13 EVGA http://www.evga.com/products/featured.asp These are low cost alternatives readily available at hardware stores like Fry's. They have many dual-monitor models, but I have not seen any model that supports 4 (or more) monitors. If you want to support more monitors on the same video card (instead of using 2 cards), you may need to look at other makes such as Matrox. A dual-monitor video card from PNY or EVGA for the PCIeX16 bus is about $50 these days. Not the high-end game graphics stuff but plenty good for price charts. If you use more than 1 video card, from my experience it's best to use cards from the same manufacturer because of compatibility issues. I had tried mixing PNY and EVGA cards on the same box once and it didn't work.
Another cheap card (<$50) is Radeon 5450. Passive cooling and low power consumption. It can drive 3 monitors per card. If you get one of those, be sure to get one without fan (noise) and with a DisplayPort output. Only those models with atleast 1 Displayport output can do 3 monitors, the others only 2. The downside compared to more expensive cards like the Matrox Triplehead is that this setup will require 3 different connectors to work (displayport, vga and dvi). P.S.: in order to get 3 monitors working, you need 1 displayport-capable monitor. Passive dp->dvi adapters wont work, and the expensive active adapters cost twice the price of the card.