The best answer to your question is to ask your tax preparer or accountant for his rule on this.
The IRS rules on this are very vague (something like "anything that extends the life or adds value to the equipment should be an improvement"). However, most accountants will give you a more practical rule of thumb (such as "if it's over $X then treat it as an improvement" (every accountant has a different number for $X)).
If you're not sure put it in an expense accountant (such as "Maintenance Expense") and at the end of the year print out a general ledger detail report for this account and give it to your tax preparer and have him tell you if there's anything that should be an improvement.
If he finds things that should be an improvement go back to Payments and Purchases and Purchase the improvement and instead of writing a check go to the Other Expense area and choose the Expense Account (such as Maintenance) and enter the amount as a negative number. This will result in the amount being taken out of maintenance and put into improvements.