Build; saves a lot of money, in the long-run.
"pre-built computers are more "basic" and have little room for expandability though are perfect for internet browsing and LIGHT gaming."
Totally depends where you shop for a pre-built rig, and the sort of rig you buy; there are heaps of rigs that can more than hold their own, are upgradable, and expandable, without needed to purchase an Alienware.
LewisG1, if you do end up getting a pre-built rig, it could be easier to build on it, but I ultimately do not recommend this, as you'll just spend more and defeat the purpose of buying the thing in the first place; you'll have spent cash on stuff you don't want, have bought gear, and eventually you'll end up replacing the entire thing. I bought a rig a few months back offa Amazon, and it needed so much done to it (even though it was new, it wasn't made well, so I got stuck in) to get what I want out of a rig, so, I ended up starting from scratch and building my now current rig! Honestly, I wish I had just built one from scratch before the whole kafuffle; I would've saved plenty or cash and hassle. Thankfully, I rebuilt the previous rig back up again, changed it a little, and my dad's now using it in his study, so it worked out OK, for me, in the end.
"Most pre-builds normally do not have a graphics card that can handle graphical intense games unless the pre-build is a top end model like the dell xps or alienware"
I find this to be very common with in-store rigs, such as ones you'd find in Curry's shops, and a lot of them in even PC World (in-store, on display) are without the good stuff. When you look for a "gaming computer" in-store, it seems like you have the choice of nothing or super expensive. Perhaps it's different in other places, but here in England, it's all I see. This is why I shop online for computer things. With these rigs you're also likely to be caught out with a MicroATX motherboard, which stunts growth, and there's the tiny PCs which a lot of people will not realise how much growth they stunt. I find in-store prices to be overly bloated, which is an opinion I'm sure is very much shared amongst builders familiar with what the real prices are.
Ultimately, if you build, you get the parts you want, at prices you can juggle, with the added bonus of the satisfaction, because at the end of the day, you can look at that rig sitting proud in-front of you, and say to yourself "I built that."