Hardware power has nothing to do with the number of bytes in your ASN.
You should choose your hardware platform based on the number of routes you will handle.
I recommend either a x86 server with 2GB ram or more or an BR1000 with 2GB ram, if you plan to receive the full routing table.
For one full IPv4 feed (312377 routes) & one full IPv6 feed (2728 routes), my x86 box only uses 261 MBytes of RAM. My recommendations would be to use atleast 512MB of ram for only 1 peer, 1GB for 2-3 peers. But ram is cheap, so put a 2GB in your RB1000 or RB1100 to get 1.5GB or 4GB in a x86 router to get 3-3.5GB (RouterOS is only 32-bit).
You might be able to use a rb450G (256MB of RAM) or rb800 (256MB), you would need to use route filters to cut down the number of routes. Dropping all /24 and longer routes should remove almost 50% of a full BGP feed. Also disable or uninstall all RouterOS extra packages: hotspot, ppp, wireless, dhcp, etc. A misconfiguration of your filters could crash your router, so more RAM would be preferred.
A rb450 only has 32MB of RAM, it would be impossible to get a enough routes from both ISP's to let BGP work to it's full potential. It might also be impossible to filter the number of routes down far enough to keep the router from running out of RAM and crashing or dropping the peer(s). Only a routerboard with 16MB of ram could be worse for BGP peering.