If boxing is your primary use of this PC, I'll echo what Ualaa said on 2 points:
1) Get a 2600k instead of a 970. You really don't want to buy 2 year old technology.
2) Get a more powerful single video card, rather than 2 lesser cards. SLI/Crossfire will not help with boxing. Even if you were planning to have each card drive separate monitors, I believe you'll get better results from a single card hooked up to both monitors, especially if you're getting a more powerful card.