Here is a build. I made a number of compromises to meet all of your requirements rather than neglecting any of them too badly. I'm sure that any number of tweaks can be made, especially since you indicate you are willing to spend more. I did go slightly over, but I also included most of the small stuff like KB, mouse, and speakers. I didn't add a surge protector, UPS, fax modem, printer, and any additional LAN cables or a router.
I chose this case because it is simple and clean, and has a tool-free design. Air flow is decent. You get a lot for the low price.
COOLER MASTER Elite 330 RC-330-KKN1-GP Black SECC ATX Mid Tower Computer Case - Retail
Model #: RC-330-KKN1-GP
Item #: N82E16811119115
http://www.newegg.com/product/product.asp?item=N82E16811119115
$39.99 $39.99
I like Gigabyte mobos. They have a solid reputation. The P35 chipset offers the highest currently available future-resistance.
GIGABYTE GA-P35-DS3R LGA 775 Intel P35 ATX Intel Motherboard - Retail
Model #: GA-P35-DS3R
Item #: N82E16813128050
http://www.newegg.com/product/product.asp?item=N82E16813128050
$129.99 $129.99
The video card was a little more difficult. An 8800 GTS would have been nice, but I thought would be too over budget. On the other hand, a 7600GT would have been cheaper, but probably would not have been adequate for a 20" widescreen at its native resolution. I chose the x1950Pro because it should handle even newer games fairly well, and this HIS card because it has a cooler which will exhaust most of its heat out of the case.
HIS Hightech H195PRQT256DDN-R-V2 Radeon X1950PRO 256MB 256-bit GDDR3 PCI Express x16 HDCP Ready CrossFire Supported Video Card - Retail
Model #: H195PRQT256DDN-R-V2
Item #: N82E16814161092
http://www.newegg.com/product/product.asp?item=N82E16814161092
$169.99 $169.99
Corsair has a solid reputation for PSU quality, and this one should meet your needs. If you decide you want better video, it will also handle an 8800GTS. You might want the 600W model for a 8800GTX.
CORSAIR CMPSU-520HX ATX12V v2.2 and EPS12V 2.91 520W Power Supply - Retail
Model #: CMPSU-520HX
Item #: N82E16817139001
http://www.newegg.com/product/product.asp?item=N82E16817139001
$139.99 -$15.00 Instant $124.99
My choice of processor was somewhat arbitrary. A Q6600 would have been well over budget for questionable current benefit; you can always get one later. I didn't think the e4500 was sufficiently cheaper to warrant the performance hit.
Intel Core 2 Duo E6750 Conroe 2.66GHz LGA 775 Processor Model BX80557E6750 - Retail
Model #: BX80557E6750
Item #: N82E16819115029
http://www.newegg.com/product/product.asp?item=N82E16819115029
$211.99 $211.99
The RAM was also a little arbitrary. Crucial, Geil, G.Skill, and others all make decent RAM, but I've been reading a lot about Super Talent around here recently, so I picked this.
SUPER TALENT 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model T8UX2GC5 - Retail
Model #: T8UX2GC5
Item #: N82E16820609226
http://www.newegg.com/product/product.asp?item=N82E16820609226
$83.99 $83.99
This hard drive offers more capacity than you need, but the price difference between it and a smaller drive is just too small for the added capacity.
Seagate Barracuda 7200.10 ST3320620AS (Perpendicular Recording Technology) 320GB 7200 RPM SATA 3.0Gb/s Hard Drive - OEM
Model #: ST3320620AS
Item #: N82E16822148140
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148140
$79.99 $79.99
The keyboard choice is another arbitrary one, but I've used these on some recent builds, and they've been fine. One of them was for my mom and she likes it well enough; it is also spill-resistant.
Microsoft ZG6-00006 OEM Black PS/2 Wired Standard Keyboard 500 - Retail
Model #: ZG6-00006
Item #: N82E16823109164
http://www.newegg.com/product/product.asp?item=N82E16823109164
$14.99 -$3.00 Instant $11.99
I chose this 20" widescreen, not only for gaming but also for the spreadsheets and other programs you say you'll be using. If you want to save money, drop back to a 19" and then a 7600GT video card might be adequate. I think you'll like this one though; if I were in the market I might choose this one myself. If you are concerned about a few dead pixels, you can always buy it locally where an exchange would be easier.
ViewSonic Optiquest Series Q20WB Black 20" 5ms DVI Widescreen LCD Monitor - Retail
Model #: Q20WB
Item #: N82E16824116075
http://www.newegg.com/product/product.asp?item=N82E16824116075
$219.99 -$20.00 Instant $199.99
The mouse is also a big personal item. I really like my G5, but thought that was a little extravagant for this build. I do think you'll like the tilting scroll wheel on this one though.
Microsoft IntelliMouse Explorer Platinum B75-00092 2-Tone 5 Buttons Tilt Wheel USB + PS/2 Optical - OEM
Model #: B75-00092
Item #: N82E16826105161
http://www.newegg.com/product/product.asp?item=N82E16826105161
$21.99 $21.99
I chose the more expensive Retail package for the DVD drive because for only about a $5 premium, it includes all the software it needs to watch movies as well as just burn data disks.
LITE-ON LightScribe Black 20X DVD±R Burner with 12X DVD-RAM write Black IDE Model LH-20A1H-186 - Retail
Model #: LH-20A1H-186
Item #: N82E16827106045
http://www.newegg.com/Product/Product.aspx?Item=N82E16827106045
$37.99 $37.99
I don't trust Vista. More people lately have been saying their Vista systems have been okay, but it is not a chance I would take in your place, on a machine for work as well as gaming. Do you feel lucky? I don't think the benefits (?) outweigh the risks. Get XP and have done with it.
Microsoft Windows XP Professional With SP2B 1 Pack - OEM
Model #: E85-04741
Item #: N82E16832116059
http://www.newegg.com/product/product.asp?item=N82E16832116059
$139.99 $139.99
Perhaps not needed, but I threw in this additional 120mm case fan. The Centurion Elite will take this up front for additional cooling. If you go with the video card I chose, that exhausts its heat, you could skip this.
Update Antec 761345-75120-9 120mm Case Fan - Retail
Model #: 761345-75120-9
Item #: N82E16835209005
http://www.newegg.com/product/product.asp?item=N82E16835209005
$8.99 $8.99
Being deaf in one ear, I'm not the one to consult on speakers, but these looked adequate until you choose to upgrade. You mentioned music, so I'm certain you'll want a subwoofer.
Logitech X-230 32 watts RMS 2.1 Black Speaker System - OEM
Model #: 970123-1403
Item #: N82E16836121123
http://www.newegg.com/product/product.asp?item=N82E16836121123
$38.99 $38.99
Subtotal: $1,300.86
And there you have it. Some people will note the omission of an aftermarket CPU cooler, but overclocking was not mentioned, so the stock should be fine. Other choices are possible; you may have strong preferences for certain things that weren't described in the OP. It would be easy to spend more money on the case and a better video card. You should get a surge protector, and may want a UPS. You may want a printer, and maybe LAN hardware so you can share it with your laptop. You could save money on the mobo. I'm sure others will post some good ideas too.