well, if you're already using the 8800 gtx you probably won't notice a big difference. The 9800 gtx performs slightly better at low to medium resolutions and for gaming it tends to shine when antialiasing is turned on.
The main advantages though are that it runs cooler and consumes less power than the 8800gtx, is about $100 cheaper and can be used in SLI setup up to three cards.
My personal experience with it is that I can get away with really pushing high settings on most games without getting bad framerates, so I can't complain there. In most of the graphics tests in 3Dmark06 my framerates stayed at or above 100 fps, except for a couple of times where it dropped to around 60.
If you're going to go with 2-card SLI, your best choice there is obviously the 2 9800 gtx cards. However, that will wind up running you about $700 u.s., something to the tune of 400 british pounds if I'm not mistaken. Also, they take up 2 expansion slots which means that you will be blocking at least 1 of your pci slots. Also you will need a psu with 4 6-pin pci-e connectors.
2x 8800 gt will deliver about the same performance as a 9800 gx2, but will not take up too much space or require a hefty power supply.
I've never heard of being able to install additional video cards without them being part of the SLI setup. Is this something you've done before? Especially different video cards with different drivers. If you want dual monitors, you can simply set that up with even just one video card. All of the cards you mentioned have dual dvi outputs and if you're really desperate you can use a usb to dvi adapter for your second monitor. There is no reason that you can't run a dual monitor setup with just one video card.
Cooler Master HAF 922 case
Thermaltake 750 watt modular psu
MSI P67A-C43 board
8 GB Corsair XMS DDR3 1300
Intel core i5 Sandy Bridge quad core@ 3.3 ghz
MSI GTX 570
WD 2 TB HDD
64-bit Windows 7
Rosewill USB Wirless Adapter
LG Blu-Ray DVD Combo Drive
I'm a PC!