How Do I Make a Futon Work as a Bed?

Recently my boyfriend and I decided that two people sleeping on a full-sized mattress wasn't going to work out. We wanted to find the cheapest solution in making the upgrade to a queen. Luckily we found a great used platform style futon frame with mattress for only $50. However, upon bringing it home we noticed that it's 6" shorter than a queen-sized mattress!

Not only is the mattress too short, it's also too uncomfortable. I quickly ordered a 4" memory foam mattress pad from Overstock.com but I'm not sure that'll be enough. To top it off, the frame also seems way too low. Although we love platform beds, this futon frame only sits 3" off the ground.

Should I learn to make it work by keeping the mattress and frame or should I splurge for something more comfortable? Do you have any tips for making a futon more accommodating as a bed?

Image: Flickr user robinsan, licensed for use under Creative Commons.