I want to connect wired Mikrotik system to APs in bridge mode. About 40-50 uses will connect to hotspot. Each wit his own account.

What is better using RB450 , RB750 or PC with 1GB RAM and 2.0 P4 processor?

My experience with the RB750 is that it maxes out at about 10MB/s. It is probably much more if you use the built in switch chip instead of having the CPU do the bridging. I don't see the need to buy a RB450 any more unless you want a serial port and metal case. The PC Wont have any problem but will use more power and not as convenient.

RB750 comes with a L4 license, so user manager can only have 20 users active at any time. The license (L5) that comes with the RB450/RB450G can have 50 active users at the same time.

The L5 license itself cost more than the RB450 device, so the best deal would be the RB450/RB450G RouterBOARD.

RB750 comes with a L4 license, so user manager can only have 20 users active at any time. The license (L5) that comes with the RB450/RB450G can have 50 active users at the same time.

The L5 license itself cost more than the RB450 device, so the best deal would be the RB450/RB450G RouterBOARD.

L4 allow 200 users which is more than enough for me
L5 allow 500 user

http://wiki.mikrotik.com/wiki/License_levels

the only reason that I as ask about PC because I thought that RB750 or RB450 processor are not enough.

I think I will buy RB750