With Gingko Biloba, you should probably limit yourself to 20 mg at a pop (at least to start -- you can always increase in small amounts). Take that twice per day. Gingko in too large a quantity at one sudden time could cause a person to spasm or even convulse due to its dilation effect on the blood vessels.
No, 500mg vitamin C is not too much, but you should try to spread it out throughout the day.
Remember, there are a lot of issues behind impotence. Diet is a huge part of the problem. If you're not eating a clean and healthy Diet (meaning you're eating lots of meat and hardly any fruits or veggies), you're going to keep having problems even with supplements like Gingko, vitamin C, ginsengs, and so on.
If you're overweight, you're at risk of impotence since it's likely your diet is incorrect. Furthermore, as you lose weight, you will release excess estrogen (from the fat deposits), and this will compound the problem until the extra weight is gone from your body.
Sleep is a factor too. If you're overly tired, it's quite possible you don't have the necessary sex energy.
The key to maintaining during sex is being comfortable with the situation and having enough free testosterone in your body (along with enough NO [nitrogen oxide] to help maintain the erection process). I'd suggest you look up the erection process and learn more from there. You might just solve your own problem, and it's entirely possible you're just "nervous."