Depends on how well established they are. If all of them get below 50 uniques a day, a VPS should be fine I guess. If they're medium sized (200-300 uniques a day), a dedicated server with 1 GB RAM and a dual core processor should be fine. If large established proxies, I'd suggest getting multiple 1 gig RAM, dual core processor servers rather than a single 2 gig+ RAM server
Only if the proxies are low-traffic ones you can do with a 2GB dedicated server. In general, 2GB are not too much - normally gets so easily "eaten" by the 50 proxies.
Dual with 2-4 gb ram can handle this load.. may be little up.. dont worry about resource start step by step keep increasing resource.. you ll see the destination !