The key questions have to be what are you doing with the pictures and what subjects are you taking?
If pictures never go beyond A4 size and the subjects are generally static and easy to capture then sounds like a second hand 10D will serve you as well as anything else. More megapixels will just mean larger files.
On the other hand, if you're taking photos of motorsport (for example) you might be better off getting something with a faster AF system (for example - I haven't used a 10D enough to know how it performs, but I wouldn't mind betting it's not class leading). If you just take snapshots and landscapes that won't be an advantage.
If you enlarge pictures to poster size or often have to crop them heavily, more megapixels won't go amiss.
Also bear in mind that a full frame camera will affect the field of view of your existing lenses and might make them an inappropriate choice. I would say a full frame DSLR is an advantage only in lighting conditions that really challenge a small format DSLR with respect to noise. i.e. low light, or where image quality has to be the best attainable.
As already said, lenses can make the biggest difference to your results, depending on your choice of subject, so blowing your whole budget on a body may not be a wise move.
Why don't you look at your current results from the 10D and try to see if you can identify its' weaknesses? If you can't then another one will be fine IMHO.
Or.. On another tack, I find built-in flashes on DSLRs to be universally sh1te, so get yourself a nice external flash that gives you a bit more flexibility in how you light the subject (by bouncing, remoting it from the body, etc.) and ignore the fact that the one on your body is bust.

Kevin