I just want to clarify that oldyeller was right that scsi, sata (150 or 300, sometimes referred to as sata I & II) will not work in your system, but he must have mistyped when referring to pata.
Pata (parallel ATA) is just what you have in your system. Do not worry if you find a drive you like that is ATA 133, as it is backwards compatible with ATA 100.
Your link to the drive you were proposing was there, but got corrupted by the [/url] on the end. Here is a clean link in case anyone following wants to look:
http://www.newegg.com/Product/Product.asp?Item=N82E16822148137
(i.e. Seagate Barracuda 7200.10 ST3400620A 400GB 7200 RPM 16MB Cache IDE Ultra ATA100 Hard Drive for $119.99).
That will work with the type of connector you have. You did not indicate what OS or what size hard drive you are currently using. Just be aware that IF your system does not see all 400 gb at first that you will have to take another step or two to get all of it recognized. This can be an issue with some older computers.
The Seagate website says that this drive uses on average 13 watts during operation, but does not mention the maximum. I mention this because your system has a 250w power supply, which is quite low (that is normal for Dell). While I would hope that you have enough extra power capacity to accomodate another drive, that depends on what else your system is powering and how much extra capacity Dell designed into it. Hopefully, as it was designed with an empty drive bay, it has the power to support it. I just wanted to let you know in case you start experiencing any out of the ordinary problems after upgrading it may be related to this.
Do not let the power supply scare you off from upgrading, as it is not likely to be a problem and, even if it were, you could either (1) buy an external enclosure (appx $30-40) and use one of your hard drives as an external drive or (2) move everything over to the larger drive and not use the old one.
This leads me to another point. When I looked up your system I was seeing references to 40 & 80 gb drives. If you still have your original drive then I would strongly suggest that rather than add the new drive as secondary storage, you transfer your OS over to the new drive and either put the original drive in as secondary storage or leave it out altogether. I am suggesting this because even though both are ATA 100, the new drive will provide a performance increase for your OS.
As the drive you are looking at is OEM, just be aware that you will need to go to the Seagate web site to download the software to set up your new drive as OEM drives do not come with installation disks.
Good luck.