You should <not> encounter any problems upgrading the hard drive. However, Dell is known to screw with their BIOS and make certain things incompatible. (for example, I replaced the motherboard in my mom's laptop once and the CPU was no longer compatible... i had to purchase a new CPU that was identical to the old one, but had a different model number.)
for example, Dell wants $410 to change the config from a 60GB 5400rpm to a 100GB 7200rpm. I can buy a new 100GB 7200rpm Travelstar 7K100 for $245.
/me nods. Ever heard of profit margins?