No problem,sourav_1988.
What I meant by the statement was PCI Express has different versions or rather ,it has many revisions.Quoting from Wikipedia,there are 4 revisions of the original PCI Express V1.0a--->v1.1,v2.0,v2.1 and v3.0.You can read more about PCI -E here-
http://en.wikipedia.org/wiki/PCI_Express
if you want.However,the crucial part in your query is the fact that PCI-E revsions are generally backwards compatible,meaning ,say,a 2.1 card can (not should)
work in a PCI-E v1.0a Motherboard x16 lane,though I have heard people saying that there is some loss of performance,although I am not sure of that.So,compatibility wise,3.0 is backwards compatible with a 2.1 or even a 2.0 Motherboard and so on.But,and this is important,
while purchasing my 6850,I was told that there can be problems getting my 2.1 PCI-E card to work on my MSI MS 7525 ,which,according to CPU-Z,a software for knowing your hardware,is reported as PCI Express only,which I assume is V1.0.So,first thing you should do is check your Motherboard's PCI-E X16 lane version.I looked it up on Google,and it is probably v1.0.So,it's better to download CPU-Z and find out the exact version of your Motherboard,It will be under the 'mainboard' tab in CPU-z.
IF your Motherboard supports PCI-E v3.0,then you should go for the 7770,not the 7750.It will be faster than the 7750,consume less power than the 6850 (80w) and has newer tech than the 6850.Plus ,it's cheaper than the 6850,thoguh I am not sure,you have to check.
But you have to purchase a PSU for that.A corsair CX430V2 from Flipkart will do fine.And do you have a monitor that is 1024x768 by default,or do you force that resolution?If you have/plan to buy a higher resolution monitor,go for the HD 6850.
If not,the 7770.
So,before buying GPU and PSU,make sure of-
a)PCI-E x16 lane version.(most of the times newer cards are backwards compatible with older motherboards)
b)amps on the 12v rail in the PSU(you should be fine with the corsair)
c)Graphic card size(physical).
d)Number of PCI-E power connectors from the PSU to the GPU.
These(a,b,c and d) are some points you must keep in mind while buying a new GPU/PSU.