Whoa! As crazy as that sounds, that's almost typical for people on tandems. They regularly go 60+ MPH on extended downhills. Me? I don't have the guts to do that on a vehicle with contact patches the size of a dime. Here's a shot of me on a typical descent:
It's really getting kinda ridiculous. I am seriously almost always the last one to the bottom of a descent.
I don't have any particular light recommendations, but there are a lot of good ones cheap. A couple of years ago, a company called MagicShine sold a 700 lumen light that cost only about $80. It was a widely-sold Chinese knockoff of a rather expensive light by another manufacturer. They seemed too good to be true, and for many, they were. There were charger/battery issues that involved the risk of fire. I bought one and had no problems, though.
Payback being what it is, the knockoff-er later became the knockoff-ee, and other started selling the same light for even less ... about $40, including a head mount, bike mount and even a rear light. Here is one example:http://www.amazon.com/1600l-Ultrabright ... ycle+light
I can't vouch for the serviceability of the light, but for $40, it may be worth a try.
I also have an older version of this light:http://www.exposurelights.com/product/0 ... Ly-t_Vs6WY
As you can see, much more expensive. But 100% reliable and durable (necessary when you're out there on your own at 2AM). A friend of mine actually had her light run over by a car, and it barely left a mark. Mine will go over 24 hours on the low setting (which is more than enough to see at normal speeds), which is pretty sweet.
I also ride with this on my helmet:http://www.princetontec.com/eos-bike
There are brighter LED lights out there than this one, but it's very durable and does the trick. It is bright enough to be a reasonable backup, and takes AAA batteries, so you can carry or purchase extra batteries if need be.