am planning to deploy a service for 300 users hotspots concurrent users with 450g.l will be using simple queue and total bandwidth will be 30-40 mbps.
Can it handle it.Any better idea will also be appreciated.
thanks
Hello friend,
In theory, at least, the 450G should be able to handle that.
CAVEAT If you try to add many firewall rules, L7 filtering and/or mangle, this loads the CPU greatly and you may find yourself bogged down.
I use 450Gs with heavy filtering, mangle and L7 rules. The largest group I have on there is about 120 users at once, and my processor speed is up there around 47-60%. If I get someone fooling around trying to get many IPs to the same MAC, or someone trying to get more P2P connections through than I allow, the processor speed can go up to 100%
You should figure on some more usage of the processor as well if you plan to have User Manager operational on the box.
In all honesty, for robustness in your planned venture and for expandability, I would go with an RB1000/RB1100 or, at a minimum, a dual processor PC-based Mikrotik.
Good luck! 
I dont have specific experience with a setup like that, but I would say it depends on what you are doing for queuing. I have customers with 20ishmbit connections and a RB450g can handle a modest firewall ruleset (~90-100 rules, very few jumps) and a couple simple queues. If you used hotspot simple queues and lots of concurrent sessions, I can see it being bogged down a lot.