Xbox 360 HDD Upgrade Without Hard or Softmod?

First off, yes I know it's a long subject line but I wanted to be specific.

Second, if that was covered somewhere already I'm sorry I searched but never found anything that tackled my specific topic.

Ok time for the issue at hand, I'm on the edge here with wanting to purchase a new 360 and have done a lot of research into the hardware side of the 360 and found that all of the models are identical now aside from having a HDD and the size of it. If possible I would like to purchase an Arcade model and purchase my own HDD to put into the system because as everybody knows MS is overcharging for a 120gig drive by at least $50.

So is there a way to purchase a HDD to put into the 360 and have it function as if it was the pre-installed factory version without soft or hard modding the system?

I know you can soft or hard mod the system pending your own preference but this can impact your ability to use the Xbox Live functionality or maybe it doesn't anymore? Please keep in mind I am a noob when it comes to this area of customization but am by no means tech handicapped. Also I am very insistant on not wanting to void my warranty because let's face it the 360 hasn't had a strong track record of reliability with RRoD and the latest E74 error.

Thank you all in advance for any help and feedback.