best way and its cheap, is a matrox g400 or g450.. 70 bucks man.. i love it.. couldnt live with out it.. (of course i am right now due to space constraints) but its the greatest thing since sliced bread.
Even though a PC supports only one AGP card, all PCI (and ISA) slots can also host video cards. Though, I'd doubt you'd wanna use an old ISA video card.
You can usually find a decent 2 to 8 meg PCI card in most computer vendors - or - you can probably steal one of that old pentium 133 that collects dust in the garage.
You may use up to as many PCI slots you have for video cards.
so, 1 agp video dual out vidcard plus 5 PCI video cards + 7 monitors = a wall of video.
most people have their main vid card (whatever, radeon, geforce3, voodoo5) and another cheapo 4 meg pci card to have dual monitor.
Oh, your OS has to support it as well (as most do anyways these days)