Hi btan219,
It's been mentioned in the other posts, but it bears repeating - if you RE-take a practice CAT (regardless of whether you're on a different computer or not), you will see some repeat questions. Each CAT has a limited "pool" of questions, so taking it a second or third time won't get you an entirely new set of questions to work with (regardless of what you're getting correct and incorrect). By re-using a CAT, you ultimately end up with a score that is NOT realistic. Thus, you have to be real clear on the "reality" of the situation.
1) If you want a realistic assessment of your skills, then you need to take a NEW, FULL-LENGTH practice CAT, under test-like conditions. The more you deviate from 'reality', the more your score is likely to be 'off' (and in many cases, higher than it should be). As a result, you won't know your 'weak areas' and you won't know what to focus on as you continue to study.
2) If you just want to do more practice problems (or retry some prior questions), then retaking a CAT is fine, as long as you realize that the end score results are NOT accurate.
While there are plenty of study resources to choose from (some are more expensive than others), you shouldn't be focused on approaching this whole process in a 'cheap' way. The GMAT is a significant factor in Business School applications (for many Schools, it's the most important part of the application), so trying to save money on this part of the process might actually derail all of your plans. Studying for the GMAT is one part of a larger 'investment' in your future - if you invest properly, then you'll likely be rewarded.
GMAT assassins aren't born, they're made,
Rich