In a nutshell...
The kit should provide useful s/w for ease of install and/or ultilies to proceed forward providing some ready default setup or parameters to select from, rather than finding out on your own. Minor h/w and cable should also be included, plus maybe printed "quick start" material. If you haven't done this or familiar with adding h/w, etc., it pays for itself and added cost is low. While you can get s/w and cable elsewhere, it already there as a kit. Of course that's up to you.
As always proving details of what you have and current setup allows us to better answer your question. The banter just to get the basics becomes a task unto itself, so it helps to do so up front, what you can provide. We have no clue otherwise. If your PC fits the general description I provided, it would have been "a go". However, you didn't offer if you visited the websites I suggested, which many HD makers provide to countless new buyers which on top of anything here is added knowledge. So what size HD do you have? The reason, I offered 200Gb because its bigger than 120Gb and would have addressed the issue of 127Gb OS limits and needn't worry, as the fix is already present. The max. addressable limit patch would have been part of XP later SP's, but XP can sometimes be difficult. Do the Dell bios update so the bios becomes ready and isn't a issue. Under the bios setting, verify that "LBA" is enabled to address big drives(probably is). Also being a Dell PC, which GX620 model is it, there is at least three(3) versions of that model which also relates to the upgrade can be handled. These vary in one way or another besides size.