This should be in the hardware/software forum, plus there are a lot of other similiar hardware discussions over there.
Here's a nice gaming rig. I'll only throw in a few brand names because if you're not tied to one specific item, you can find equivalent ones in package deals for less.
Cpu - socket 939 Athlon 64. Get the best you can, but an A64 3800 "Venice" or 4000 "San Diego" would be great choices for under $400. Any faster and you'll spend double for 10-20% speedup.
If you get the retail cpu, you won't have to worry about an aftermarket heatsink and fan. The stock AMD ones are just fine unless you want to overclock.
Motherboard - Any SLI capable socket 939 nvidia nforce4 based mobo. ASUS is good, so are ABIT and MSI. Avoid ECS unless you're on a budget and willing to experiment. I'd probably go with ASUS if I was buying now.
Memory - 1 gig (2 512 meg sticks) of high quality PC3200, PC3700, or PC4000 DDR. You can get a bit of a speedup by going for "extreme" memory and even the best stuff has dropped in price, but you can get good crucial memory for under $45 per 512 meg stick, or go for some high quality stuff for about $75 per stick. Even crucial ballistix pc3200 is only $75ish per 512 meg stick. PC4000 is about the same price so if you think you may try overclocking, get pc4000 memory instead.
Vid card - Nvidia 7800GTX is the absolute fastest right now but it'll cost over $500. Get two and run it in SLI, although I don't think SLI works too well in AH2 yet. There is also a 7800GT that is up to $100 cheaper than the GTX but it runs almost as fast. For less money, get a 6800GT for around $300 and it'll run everything pretty nice, however a 7800GT is more than twice as fast in certain games according to the reviews I've read.
Sound card - Audigy2 ZS is fairly cheap and will work. Get the OEM version for about $50-$60 if you don't want all the creative labs software, otherwise buy the one with the features you want. There is now a newer one (Audigy 3 or 4?) that is a bit better but also more expensive. You can also get a motherboard with built in sound but it may slightly reduce your framerates. If you save money by going with mobo onboard sound instead of a sound card, you can use some of that money to get a better motherboard.
Network card - use the onboard one that comes with most nforce4 motherboards. It works great and is "free".
Hard drive - Almost any will do but the Seagate ones tend to be nice and quiet. The price point to aim for is 300 gig for about $130. Go with serial ATA. Don't worry about going with RAID 0 or buying a 10,000 rpm drive. Any reasonably new 7200 rpm drive will be more than fast enough.
Monitor - entirely your choice. 19" LCDs are getting much cheaper. If you can find a coupon code, the dell 1905 LCD is supposedly pretty good for gaming and will cost between $260 and $450 depending on if you find a valid coupon code or not. Do a google or yahoo search for the codes.
Buy a new license of winXP home to go with all this. Get winXP pro if you already know what the difference is, but if you have to ask what's the diff then you're probably better off with winXP home. My game rig uses winxp home even though I'm a geek, and I don't ever wish it had XP pro.
Case - I have a Lian-Li PC60 case and I really like it. It's pricy because it's all aluminum but it's very nice. A bit lighter than a steel case, and it has removable mobo tray and removable hard drive cage plus places to put fans to blow over the hard drives. Take a look at the Lian Li PC7 and PC65 cases at
http://www.newegg.com, and do a google search for those cases for reviews. Expect to pay anywhere from $90 to $130 for a decent aluminum case.
Power supply - most cases come with only a cheap 300 or 350 watt power supply, and that's just not sufficient for reliable operation with the best hardware anymore. You want at least a high quality 400 watt PSU for the parts I've listed. PC-Power and Cooling is awesome but expensive. Antec ones are good, so are enermax. I have a thermaltake quiet purepowr 480 and it's ok. Expect to spend anywhere from $60 to $210, and I'd personally aim towards 500 watt especially if going for a 7800GT or GTX video card. As a rule of thumb, several reviewers have noticed a direct relationship between power supply weight and quality. The heavier the power supply is, the more likely it is to be good quality. There are very logical reasons why quality power supplies will usually weigh more than cheap ones, so it's something to think about.
As for your old laptop, there are a lot of things you might want to try. Set the texture size to 128. Run all the sliders down to the lowest setting. There is a visual range key, something like shift-f1, shift f2 and shift f3 (?) that can help too. Run a lower resolution. Anyhow, read through the documentation files on how to set the graphics settings for better framerate.
You should be able to put together a rig like this for under $2000 if you don't go crazy on extras. You can save $100 by going for a slower cpu, but do NOT go with a socket 754 cpu and motherboard just to save money, because you'll only save about $50 and then be unable to upgrade in the future. Do not go with an AGP motherboard and vid card, as the new vid cards are all pci-express.
