unless you absolutely must have cordless, there's practically little to no use in buying a G7 over an MX518. the 518 has a great sensor which never skips during regular play. it also has plenty of dpi in case you use a very high resolution or sensitivity. at Rs. 1400, it's probably the best VFM gaming mouse you can get in India right now after the MS Wheel Mouse Optical.
edit: oh, don't bother with the G5 either. the shape of the 518 and G5 are very similar. the dpi difference would hardly be noticeable. the G5 does come with a 500hz polling rate out of the box but you can use a downloadable application to get that on your 518 as well. of course, it would be a problem at LANs but personally after a lot of testing and comparison of 125hz vs 500hz, i saw no difference in my aim.
finally, the G5 has a laser sensor which is known to have trouble with certain surfaces. the 518 would definitely be a safer bet. like Saiyan said, throw in a nice large cloth mouse pad with the leftover cash if you want.