Question about a wedding

[hollajane]

I'm new here and love all this site has to offer I was wondering if you could help... I am shooting my first wedding on July 3rd and I'm nervous to say the least. I currently shoot portraits with my D70 and I've always been happy with the results. The wedding will be taking place in a dark church and a dark reception hall. I'm worried about how grainy most of my shots look when shooting at 1600 ISO (the highest my ISO will go). I typically will shoot with my 18-70mm or my 50mm 1.8. I have an opportunity to rent a D300 for $100. Not sure if I should or not, it's a good price but from what I've heard it's a totally different camera and I won't have time to learn that much about it first but if it improves my images drastically that's a good thing. I really wanted to rent the D90 but it's not available.

Any advice or help would be appreciated. Thanks.