Dells are nice in that they offer centralized warranty service, good prices with their regular coupons, and are generally quiet PCs (tho a friend recently bought a Dimension 2300 [or whatever IGP line is a step below the 4700] and it wasn't as quiet as I expected). For a gamer, I see two problems with a Dell:
1. proprietary PSU means difficult upgrading for power-hungry upcoming CPUs and GPUs (the R520 is rumored to be 300+M transistors!).
2. P4s are slower than A64s.
For a gamer on a budget, though, those two limitations may be beyond their horizon. In that case, I guess the ease of buying a Dell, and the good deals you can find on Dell systems (check BensBargains.net, Slickdeals.net, gotApex.com, and AnandTech's Hot Deals forum) may make it worth your while. I'd still prefer an A64 for gaming, tho, considering all of the benchmarks I've seen.
You might also want to consider front firewire and audio ports and whatnot, which you can always add to any PC with a 3.5" memory card reader + USB + 1394 + audio unit (like InWin's 530), but which Dell probably doesn't include (both in terms of firewire and audio connectors on the case, but also on the MB), which means another $30 for a combo PCI card. I know the MSI includes a rear firewire connector, and I think it also includes a firewire header. That saves you a bit of money if you do want firewire in the front, say for an iPod or external drive.
oc_enthusiasm, that Dell sounds overpriced compared to the deals I remember. I think a P4 3GHz with 512MB RAM can be closer to $400-500. I also would avoid 2x256MB, as 512MB is really the min spec for modern games. Upgrading that would require you sell both 256MB sticks. It'd be easier to start with a single 512MB stick.
Se7en, I'm a little nervous that you're already buying off of my recommendations.
I haven't really been keeping up with new system components as I have with new video cards, so I strongly suggest you check some video forums to see what new systems the gamers there are building. The MSI, despite GamePC's review as less than completely stable, does look incredibly tempting for what it offers. I was considering it to put in an Antec Aria along with the "slowest" A64 I could find and a passively-cooled video card as a /very/ upgradable office PC. Considering the Aria does have airflow problems, tho, I'd need to ensure that the MSI allows for undervolting, or at least that Cool n Quiet will work flawlessly. I'm more concerned with noise and heat (read: silence) than OCing now, so I consider core undervolting options more important than FSB/PCI multipliers and memory timing configs.
OC'ing is important for a budget build, tho. I'm running an Asus nForce1 MB that can't OC, which I bought for the same reason I'm considering the MSI: price/performance. Now that it's a couple years old, though, I was looking to upgrade it. It's lack of OCing options really limited the erformance I could extract out of it. I ended up buying a used XP 2400+ and a stick of 512MB PC2700 to respectively replace and complement my 1700+ and 512MB PC2100 stick. This is somewhat sad b/c this 2400+, according to its previous owner, is a wicked OCer. It's stuck at a mere 2GHz in this A7N266-VM, tho. I really could use the extra speed of an OC'ed CPU with a higher FSB for games like HL2.
So, think of the (near) future when you're saving money now. An extra $20-40 on a MB now may pay off with a much more flexible system a couple years from now. Keep in mind that CPU advances have been relatively slow for the past couple years. I expect dual-cores to speed things up again soon.
BTW, if you're interested in a quiet computer, definitely give SilentPCReview and its forums a look.