As I know the team of ArcBlock is discuss we can buy max 2ETH or 5ETH. For me I want to buy 5ETH for this because I think it have huge future like ETH. 
Any ICOs that have been so subscribed so as that they have a limited allocation for whitelisted participants have all performed well after ICO (at least that I know of). Therefore even if it means selling other holdings I would go for 5eth and you'll be able to turn a quick profit post ICO if you want to buy back your previous holdings.