I'm just curious: Is there a big difference (performance-wise) between a 5400 RPM drive and 7200 RPM drive?
If all else being equal (cache size, number of platters, etc.) then yes, there is a noticeable difference that can be perceived without the use of benchmarking utilities.
I know there will be some because faster is usually better, but am curious if it is worth the extra money to buy a 7200 RPM drive versus 5400 RPM drive.
Only you can answer that as it comes down to two choices: would you rather have longer notebook battery life, or would you rather see more system performance? Personally, for notebook use, I prefer 5,400RPM hard drives mainly due to the fact that they consume less power. 7,200RPM hard drives perform better but they consume much more power than their 5,400RPM counterparts. It's whatever you feel is more important for your laptop needs.
Also, is there any problem using a laptop (2.5") SATA drive in a desktop computer (I have a 2.5-to-3.5 adaptor and a spare drive).
Nope, there is no problem using laptop hard drives in a desktop solution. However, you need to understand that laptop HDD capacities cap out at 500GB, so you aren't going to see the insane capacities that Seagate and Western Digital have acheived in 3.5" hard drives in 2.5" HDD formfactor.