Depending on how rich "quite rich" is, you may be able to qualify for an investor visa in a US company. If this is the high percentage option for you, start talking to an immigration lawyer ASAP.
If that is not an option, suffice it to say that the difficulty of getting a legitimate visa into the US is probably the biggest single challenge you'll face up to Series A or so. (Believe me, I sympathize.)
Not sure if you would want to put in $500k just for the sake of being in the Valley.. Alot of non-valley startups doing very well too. If you really need the connections, best way is to make a kickass product and the investors will come knocking.
Probably start with the hard grind - join a startup that offers a h1b, then go from there.
Just google for EB5 investor visa and start contacting the law firms!
If a US company or person is a partner in your company, then you may be able to get a L1 visa. I guess this can also work out if you have a US based investor, but I recommend to confirm from an attorney.