If you are comfortable opening up the case and getting your hands inside (as you had to do to install the RAM), then adding a hard drive should not be a problem for you. Well, that's not to say that it *can't* or *won't* be a problem, just that there's a nearly 100% certainty that folks here can talk you though whatever you encounter--as long as you're up for it. If you don't mind learning something new and maybe spending a little bit of time, then by all means, go for it.
Now, as far as the size of a new hard drive goes: the only limit I can think of is that depending on your motherboard and whichever type of controllers you have (ATA vs SATA mostly) you *might* have limitations on the size of drive that you can use.
Just make sure, before you order anything, that you've asked and we've answered enough questions so that you end up with a compatible drive for your system.
First step: verify that the link below is describing your computer, because if so, that provides us here with almost any info we'd need to advise.
http://support.dell.com/support/edocs/systems/dim3000/en/SM/specs.htm#wp1075776a couple more useful links; you can look these over to see how simple (or not) it seems to you
http://support.dell.com/support/edocs/systems/dim3000/en/SM/parts.htm#wp1043338http://support.dell.com/support/edocs/systems/dim3000/en/SM/index.htmedit to add more info:
read this thread for some other info
http://en.community.dell.com/forums/p/19251204/19406067.aspxAnd notice the part about the cable; most of the IDE cables have 2 connectors (for 2 hard drives) but check the cable that came with your system to see if it has the cable you need (if you only have 1 hard drive currently installed, then you'll notice an unused connector on the cable, which you could use for a 2nd (slave) hard drive (as suggested by the poster above) so you can keep your current drive and just add one for extra storage space (avoids having to clone/copy the current drive).
It seems that there might be a bracket you'd need to buy to properly secure a 2nd drive (although if you're like me, I just let them sit wherever they can fit, lol--as long as they're not touching something they shouldn't and as long as the computer doesn't get jostled--of course it's best if the hard drive is nicely secured, so get the bracket if you can)

