That would depend on:
1. Your computer specifications
2. Your budget
3. Your monitor resolution
4. Kind of games you like to play
There is a variety of graphic cards available today. It is not necessary that any 1GB card will run all games. For eg, you may get a 9400GT from nVidia for as low as 3,000 INR, but that's a crappy card and wouldn't really serve your purpose. A GTS250 from nVidia would run many games just fine, but will be more expensive.
Therefore, give us more details as stated above, depending on which we can provide some answers.
you cant crossfire a 5850 with a 5870... you cant hack the bios to make the 5850 think its a 70 like you can with the 6 series... so dont waste your cash...
as for the op
if your looking to play games theres no such thing as best and cheap... you need to meet minimum spec for games and today the best games need a minimum of an 88gt..
so your looking at the alternatives to that model which is the 5750 or the gts 250
but both will need a minimum of a 450w psu... if you have less than that your just throwing your money away until you upgrade your psu first.
You can crossfire a 5850 with a 5870, you get a boost in performance. in most cases its about equal or less than dual 5850's but there are reports of people heavily over clocking there 5850's to beat out (by a small margin) the dual 5850's. not a great system if you are buying new but if you already have a 5870 and dont want to spend alot of cash then its an option.
i suppose an 8800GT is a decent card but its the same price as some higher end cards today such as a 5770 wich beats it out hands down. I would look into maybe SLI with some GTX 450's or 460's i think you can find them for around 100. im more of an ati guy and i keep mixing up nvidias numbers. but if you want to run multipul cards then i would go nvidia , there SLI just works better and more often than crossfire.
im not recommending he buy an 88gt. im suggesting he use it as a minimum requirement to consider. games like bf3 are using it as minimum requirement so i cant in good faith recommend a lesser power card. thus the gts250 or 5750 both of which are slightly more powerful.
as the the 5850 and 5870 crosfire its the first i have ever heard of it. so could you link that please. i like to keep up to date on these things .. personally i wouldn't recommend it as you will gimp the more powerful card by a good margin... anyway if you look around you can find xfx 5870s vapor x editions for £150 and you can get a second hand 1 from the overclockers club for less than 100 from the rite seller.
edit* actualy i found an article on crosfiring mixed cards.. with benches... gotta say im surprised that it even works but the thing that didnt surprise me is that its not as good as crossfiring 2 5850's so not only do you loose out th 15 or so percent from having a 5870 you also loose another 2 or so percent because of the mix and match...
so good if you on a budget but not so much if you can find another matching card for a low price.
but thanx for the heads up. i learned somehting today