Horizontal span with different cards

I did a search but didnt find anything useful, sorry if i dbl post this.

So i have a Sapphire 4870 1GB video card. Ive recently discovered Horizontal Spanning, but ive realized that i wouldnt want two monitors. Therefore im going to be buying another video card to add the third monitor. Question is, if i want to be cheap can i just buy a lower end 4800 card? Will it affect my gaming experience too much?

I heard that when you run three monitors you cant use the cards in CrossFire, and thats okay with me. But if i buy a lower end card, will one of my monitors have crappy quality when i run games and videos across all three monitors? Or am i just overthinking it and probably wont even be able to notice the difference?

If its a tiny difference i could just put the lower end card off to the side where i wont be staring at and wont be able to notice it even more.

Ive been doing research on it, and it only works in Window XP. and yes, i actually got it to work in XP. i read on some forums that it worked in the Beta of Windows 7, but i could not for the life of me get it to work.