Two of those cards won't run 6 displays, I'm afraid. With AMD's 5xxx series and above, they have the ability to run 2 screens using DVI/VGA/HDMI and 1, 2 or more using DisplayPort. This is to do with DisplayPort not needing a clock signal output where the others do, and AMD graphics cards can only produce two clock signals in total. The 6450's you've linked to don't have DisplayPort outputs, so you are limited to two outputs at once out of the three on the card.
You can run DVI/HDMI/VGA screens from the DisplayPort outputs on a card, but you will need an ACTIVE adapter for this, not a PASSIVE one. Passive ones need the GPU to produce a clock signal (so using up one of the two you have available, even though you're using the DisplayPort output), Active adapters produce their own clock signal so the GPU still has it's two to play with.
The only exception to this is Sapphire's FLEX range, which allow you to use 3 screens running DVI/VGA/HDMI in addition to any running on DisplayPort outputs.
With Nvidia's cards, before the 6xx series came out you could run up to 2 screens from each card. With the high end 6xx series now being based on Kepler architecture, these can run up to 4 screens per card and can use any outputs the card has so you don't need DisplayPort or adapters. Need to check the card you're looking at though as some 6xx series are based on the older Fermi architecture that is still limited to 2 screens per card.
A graphics card designed for a x16 slot WILL work in a x1 slot, but as the x1 slot is shorter you will need to either use an adapter to get it to fit, or cut off some of the card/slot to make it fit (seriously, google for fitting a x16 card in a x1 slot). Also, check there aren't any components behind the x1 slot the card will foul when inserted. Also, the card will run much slower than in a x16 slot, but if you're not gaming it won't matter.
Cheapest option would be two of these:
http://www.overclockers.co.uk/showproduct.php?prodid=GX-309-SP&groupid=701&catid=56&subcat=1985
Assuming you have two x16 slots, or a x16 and a x1 slot that you can cut the end off to make the second card fit (or cut the connectors off the card).