If you are on a stringent budget but still want quality, your choice should be Creative, I say this so because you get the best after sales service with creative, same is the case with Logitech too but their speakers are a bit pricey.
Anyways, for a 5.1 system look for
Creative SBS 580 (5.1) - Rs.3818/- in kolkata
*www.creative.com/products/product.asp?category=4&subcategory=25&product=14095
Creative Inspire 5200 (5.1) - Rs.3477/- in kolkata
*www.creative.com/products/product.asp?category=4&subcategory=25&product=55
Creative Inspire P5800 (5.1) - Rs.6031/- in kolkata
*www.creative.com/products/product.asp?category=4&subcategory=25&product=10596