I have a micro niche site http://marrychristmas2013.com want to get a good host as I have traffic of 500+ still now and hope it will go for 3000+ online at that time. So suggest me best digital ocean plan.
The only way to get an accurate answer to your question is to deploy a droplet and run some load tests. But to give you a rough idea, you can achieve millions of hits (with ease) on a WordPress site with Nginx + caching and a CDN; on a $5 cloud server.
Varnish is an HTTP accelerator and a useful tool for speeding up a server, especially during a times when there is high traffic to a site. It works by redirecting visitors to static pages whenever possible and only drawing on the server itself if there is a need for an active process. This tutorial covers installing wordpress on a LEMP stack (with nginx instead of apache), and then installing varnish.