Educate me on modern desktop computer features/minimum requirements

I’ve been contemplating going from this laptop back to a desktop for a little while now. Have had the computer since about fall 2006 (back at the very end of XP pre-Vista), and it’s still running well enough, but starting to show its age a bit. I’ve already gone through one battery that I’ve had to replace after the old one wouldn’t properly hold a charge, reformatted a time or two (once very necessarily after I got some nastily annoying virus that refused to be scrubbed back in January 2009), and generally have had to work with it in the past year to keep running efficiently enough by current standards. A crash that had it angrily beeping at me earlier this afternoon brought it to the front of my mind.

The decision to go to desktop is pretty much because I’m very rarely using the laptop as anything but a collapsible desktop, anyway. It’s basically tethered to one spot in the living room. My iPhone has replaced it for pretty nearly all portable browsing/email/entertainment needs. I’d really rather have a desktop that actually is a desktop, with proper components, ability to upgrade and replace hardware as needed, and ability to use it for playing current retail games.

But I’m far enough removed from it all that I don’t know what components/features are pretty much minimally required, what are future-proofing, and what are irrelevant to care about for a while.

For example, looking around, 4gb memory seems pretty standard now. Some retail boxes are getting sold with as few as 2, some at 8, or beyond. Now, memory is just about the easiest replacement I can make later, so I’m happy to just go with what I’ll need now, and make sure I have a mobo that will support 8 or 16 down the road. Is 4 solid for everything but the most graphic intensive tasks right now?

Or processors, which are not as easy to replace, but as far as I can tell, have pretty much reached the incremental-increase wall, and now the name of the game is the number of cores/CPUs you have going at once. I last had a desktop in the age of single-core computing, and know that initially, there was a lot of inefficiency on how software used multiple CPUs. I’m sure that’s been fixed over the years, and especially with Windows 7, but how much difference does it make? Will a dual-core suffice to keep me running for a while, or will it be pretty worthless in two or three years? Quad? i5, i7?

Video cards, power supplies, secondary cards and whatever other crap I could cram into a PCIe slot…
Basically, I want something that will keep me running for at least five years. I’m not looking to build a system, but have the ability that I could. I’d rather just be able to upgrade and evolve it over the years. I’m not looking to tweak, overclock, supercool, or otherwise fight with it. I’m looking to be able to play games, but I’m not much fan of FPSs or MMORPGs; I like dungeon crawlers, strategy RPGs, etc. I wouldn’t mind the ability to go multimedia or recruit it to use as a Media Center, but it’s hardly a priority. Basic casual computing is going to be covered by anything I buy, so it’s really not in the conversation here. And I really want to keep it as cheap as possible. $750 as a max.

So, what do I need, what should I look for if the money’s still available after covering the basics/minimums, and what can I just bother with down the road?

I have considered building a desktop and am having the same problem. The CPUs are confusing, however the charts on passmark help clarify things.

For example, an AMD Phenom II X3 has twice the score (2500) of an Athlon X2 dual core 4200 (1100). Which is sputnik to me but gives me a good ballpark estimate because I wouldn’t know that from the names.

A guy named SenorBeef wrote a thread a month or so ago talking about building desktops for decent gaming for $400~. I would PM him and ask him for advice and tell him your specs, demands and price range. He might have some good advice.