Are the Sabertooth Z77 and the Asus Maximus V Formula worth the money? I'm building a gaming computer with about a $1000 budget and they're pricey. I plan on getting an i5 3450 with a Radeon 7870.
No. The sabertooth is a mid-end Asus board with "thermal armor" and a sticker $200 higher. And, as is expected, that "armor"? Just makes the temps higher. It's a POS to be avoided at all costs.
As for the Maximus V, it's a very good board, yes... but there's no reason to buy it, especially with a budget that's not in the stratosphere.
Get a Gigabyte z77x-ud3h. It's an amazing board that got my i5-3570k to 4.4GHz @ 1.18v.
If you don't want Gigabyte, go with the AsRock z77 Extreme 4. AsRock is a subsidiary of Asus, and actually make way better boards than Asus's low-end ones.
Use that money to get yourself a 3570k - there's no reason in buying an expensive i5 and upgrading later when new CPUs on different sockets will be out. Either buy it now or buy a Pentium/i3 and cheap-o motherboard to hold you out till you can buy a nice Haswell chip.
EDIT: If what you're thinking of doing is upgrading to an i7, it's a stupid waste of money. The only difference between it and an i5 is that the i7 has hyperthreading - which doesn't help games whatsoever anyways. The i5-3570k is just about the best chip you can get for gaming right now.
Also... are you going to be overclocking? If you aren't, if you think you can use software or an auto-overclock, or if you don't know what it is, ignore everything I previously said and buy a cheap h77 chipset motherboard.