Wouldn't that (changing XP values to a string) still create some incredibly huge numbers for your level 10,000? What I was saying (that I've seen done, but don't know exactly how it was done) is to have the XP reset to 0 at each level gain so that each level requires only the same XP as the level before it, or maybe even doing it the way 3rd edition D&D does it, the amount of XP you need to reach the next level is equal to (current_level * 1000) + current_level_xp, but ...*sigh* nope, that makes the XP requirement for level 10,000 = 49,995,000,000 ...so I guess that wouldn't work either.
Maybe if you made all XP values reduced by a power of 10 and set the XP for next level = (current_level*100) + current level_xp? Let's see, that would put the XP requirement for level 10,000 at 4,999,500,000 which still doesn't quite cut it, but it's getting closer, the problem is that the code won't support XP values higher than 2,147,483,647.
Perhaps if you set your XP forumula to (current_level * 50) + current_level_xp? Let's see... oh, hey, now we're really close, level 10,000 is still just a hair too high coming in at 2,499,750,000, but using that method (having scaled XP values for mobs and skills and such down by multiplying all values by .15) you can get all the way up to level 9,268 with a mere 2,147,163,900 but you still have XP values rolling on you at levle 9,287 since the xp requirement for that level would be 2,147,627,300. So I guess you'd need to factor down it even more if your XP progression is going to remain static. (And you're in real trouble if you use the different values for each class like in the current system designed to more or less mimic 2nd edition D&D).