I'm a Fidelity only investor.
Both of those are great funds.
FUSVX (Fidelity's 500 index) has an expense ratio of 0.045% meaning you pay 45¢ for every thousand you invest in it. It is low turnover so that means it's tax efficient, and it is a broad index fund based off the S&P index.
That being said, FSTVX (Fidelity's total market index) is better. Same very low cost, but much broader range of funds (not just the S&P), so it is MUCH closer to the definition of diversified. If you have a choice, go with FSTVX, but you're not doing at all badly choosing FUXVX.
You don't mention who your 401k is with, but I'm going to assume it is with Fidelity since those are both Fido funds. While VTSAX is great and the gold standard for index investing, you will likely be charged a transaction fee if you invest in it outside of Vanguard since it is a Vanguard fund. In Fidelity's case, buying and possibly selling VTSAX inside a Fidelity 401k or tIRA will cost you $75 if I am reading the fine print correctly.
Since Fido already offers their own line of index funds (FSTVX is the equivalent to VTSAX), then I would not buy outside funds like Vanguard there. There's no point as the FSTVX is going to run the same costs, and perform similarly to Vanguard's total market index, but will be a no transaction fee option.
https://www.bogleheads.org/wiki/Fidelity^good review of Boglehead investing using Fidelity funds
And there are two schools of thought on investing in international stocks. I get why Radagast suggests adding one in, but I personally do not do so, because I agree with the line of thinking that the U.S. stock market is actually pretty well covered internationally just by the virtue of having companies that do business around the globe. And I also like only holding three funds over my entire portfolio (I have a total stock, total bond and a REIT index), adding another one just seems to be heading towards complication. Three is plenty for me. :)