Theymos has said earlier in this thread that if you donate 10 BTC to become a donator, then you can donate 40 more coins later to become a VIP. I'm not sure enough about my answer to say that 5BTC now and 5 BTC later would get you a Donator Tag. My guess would be yes, however Theymos would be able to tell you yes/no for certain much quicker than I could.

I don't see any point in donating 5 BTC now and then 5 BTC later. You won't get Donator status until you've donated a total of at least 10 BTC. It'd be better for you to earn interest on the 5 BTC somewhere until you're ready to pay 10 BTC. The forum isn't in need of money.