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One thing to keep in my while choosing a computer is the processor. You will pay a significantly larger amount of money for something you might not need. Check out this blog for quick yet very informative reasons why.
So the first question you have to ask yourself is how much you game and how much of it absolutely requires high-performance (the latest FPS games tend to be bad, casual games such as those from Popcap are non-issues, e.g.). If you genuinely don't care about gaming (or only care about it a little bit), you're about to save yourself a ton of headache. You won't likely need a dedicated (or "discrete") graphics card and CPU performance won't matter too much.
Let me say that again: CPU performance won't matter too much.
If you're running standard Windows and want to run simultaneous VMs, RAM quantity will haunt you way more than CPU performance ever will. WAY more. Get yourself a motherboard that sports a minimum of 4 DIMM slots and fill them with 4GB each and you'll be happy (or at least as happy as you're likely to get--I find that the happiness factor with a new PC lasts about 4 months before regret of some sort starts to kick in).
The one factor that WILL matter with regard to CPUs is whether they support the virtualization features that certain hypervisors prefer and/or require. On AMD, this is called SVM (or AMD-V, depending on who you talk to) and on Intel this is called VT (or VT with extra letters after it, depending on what incremental features Intel feels like putting on their Xeons these days). Here's where my AMD fanboy stripes show:
1. EVERY socket AM3 AMD CPU already supports these features. Even the ~$30 Semprons.
2. Intel's support of this feature is all over the map. You have to double-check http://ark.intel.com to verify whether VT is supported on any given processor (though to be fair, most of the Core i-Whatevers seem to).
If you're taking cert exams, then you really need to consider where your money is being spent (tests are expensive!). My suggestion:
1. Build your PC with a so-so 3-core AMD chip (like the $75 Athlon II x3 445), a decent MicroATX motherboard from a reputable vendor (like the $65 ASUS M4A78LT-M) and 16GB of RAM (like the G.SKILL DDR3-1333 4x4 kit for $99) and one (or preferably more) 7200 RPM HDDs and a decent chassis. I've taken the liberty of putting together a list of Newegg links: http://tinyurl.com/3ejc6k3 With the chassis and 2xHDD, this is a $400 box (plus shipping/tax, maybe another $40 if you really need a DVD burner).
2. Look very seriously at getting yourself a used Cisco device, preferably something with a horrible UI (who am I kidding? They all have horrible UIs :-) ) like a Cisco 1800-class switch/router (300-400 on eBay). Of course, if you don't want to bother with a network cert, then no need to bug on that.
3. Spend the rest of the money on tests and prep and lots of time on practice.
Can you make a better PC than what I described above? Absolutely. Much better, and people will argue non-stop about which processor pushes out the better frame rate in some game or uses less power or whatever. But you have to build for the problem you're solving and that doesn't need an extra 2 fps in some game.
What's your total budget for the pc? You might want to look at a 6 core Phenom II X6. I've got one and it's NIIIIIIIICE. Same price as the Core i5 4 core but it's faster by 600mhz and has, of course, 2 additional cores - which is something you'll want if you're going to run multiple VM's.
I'd budget at least $600 for the PC - get a high-quality power supply, at least 500w-800w - and a minimum of 4gb RAM - 8gb if you can swing it.