Ok, I have finally gotten down to your question, and I want to 1st ask another.

What is your budget? Do you want another desk computer? Or are you looking for a laptop? Many laptops now rival desktops, but do come with a higher price. FPS on a laptop equals serious money!

That being said, you might consider a botique computer, versus a Dell or HP. In many cases, you can get a local built computer for much less than a mass merchant system. As RNT suggested, some of the more "mass built" systems use a special MotherBoard that limits upgrades in the future.

Or they "cheat" and use an underpowered video card, (that steals power from the CPU). They may also use an integrated sound (on the motherboard) also stealing power from the CPU.

Dual and Quad core are the way to go, but be prepared for higher cost. Again, this may or may NOT be a concern, but you havn't told us what you want to spend.

Here are some base line recs for you. It'll be a "Mid-Range" and a "Entry-Level" system.

These would need to be assembled by a local computer expert, although if you really wanted to do it yourself, its not really all that hard. (seriously, and its kinda fun in a Frankenstein way!). If you take this list into a computer shop they should be able to get the parts from Newegg,com or similar places.

MID RANGE
Case- 500W ATX (W is for Watts, DONT GET LESS THAN 500W NO MATTER THE SYSTEM) $60

Processor Intel Core 2 Duo E6700 $274

Motherboard Asus PSN-E SLI (Asus is cherry, I would not go less than it. Very easy to use and lots of room. Compatible with all chasis.) $115

Memory
Corsair 2GB DDR2-800 (Dont get less than 2GB, and the spec and speed are good. All memory is NOT the same) $163

Optical Drive
Plextor PX-760A (I've used Plextor before, quiet and reliable) $36

Hard Drive
WD 250 GB 7,200RPM (you can get 10,000 RPM, but you pay out the ass for it. Not applicable for you unless you need the extra speed for your LAN parties) $60

Soundcard
Creative Labs X-Fi Xtreme Music $47 (Creative is king here. Dont go less)

VideoCard
GeForce 8800GTX 768MB $459 (DONT SKIMP. I realize this is an eye opener...more than the actual CPU, but its sole job is to make your FPS the best possible experience. Also used in any graphics application, from photo editing, video editing, or the actual windows vista operating system. If you have the cash, get upgrade to the GeForce 8800 ULTRA 768MB and link them. Cost of that option is $1,202. Seriously overkill unless you want some serious drooling from nerds at your LAN Party)

If that card is way to much, step down to the GeForce 8600 GT for $105. Ask your assembler to overclock it a bit, and you will get decent performance for the money. This is the real/main difference between the Mid and the Entry level.

The CPU on the Entry Level also uses a AMD Athlon 64 4200+ for a cost of $70.

Let me know what you think. If you don't understand anything let me know. I'll help ya!

Your on your own for a monitor. For starters, just keep what you have. Unless its dying, it should work just fine. It'll save you some money.

One final note. The boutique MAY suggest you get your operating system as a "OEM" product. If I were YOU, decline. You would be best to purchase from BB at regular price. The reason for this, OEM=ZERO support from MS. So if you have a problem with Vista/XP, and you bought the "OEM" version to save money, you will be Sh*t out of luck. Get the retail version, and you are assured of support from MS!