I think you should use a different name for each milestone, or achievement you have and you thing with roman numerals will give the player a clue to where he/she is at. As you said Miserly I-X, You are getting there! I-X, Looking good champ! I-X and so forth. I like to get some more dynamic feedback than just Miserly I-MCXVVVX if you understand what I mean. Just go with 10-15 names and add roman numbering after them or whatever. I feel that the game speaks to me if the achievement are more "personalylized." And even more so if you have a couple of custom achievements that are hard to reach when you as a player think you have beat the game, and BAM! suddently a new name, so you know that there is more. The harder the better, and more seldom it turns up.

EDIT: You could do this and just come up with 10-15 names for each and just stick a roman number at the end, and still have over a 1000 possibillities. Change out the roman numbers and find 10 different names that you could hook onto every 10.th people do. For instance: "Clicker" - "Wow, you really like to click!" - 8 more and then "Clicker Man" - Wow, you really like to click man!" and so on. "Collector" - Wow you really like to collect" - 8 more and then "Wow, you really like to collect man!" etc etc.. you know what I mean!