I'm going to be getting an XPS next month, so like you, I'm in the fun position of picking and choosing what I want.
First off all, since I see it is the default option for both of these computers, do not get the 256MB nVidia Geforce 7300LE TurboCache video card. This is a 128mb video card that gets the additional 128 mb from your system memory. Vista uses at a minimum about 600mb, so you're down nearly at a GB when you have that card.
Based on what you listed above, it looks like you're looking at a system like this:
$1,499 XPS 410
Intel® Core™2 Duo Processor E6600 (4MB L2 Cache,2.4GHz,1066 FSB)
OPERATING SYSTEM Genuine Windows Vista™ Home Premium
MEMORY 2GB Dual Channel DDR2 SDRAM at 667MHz - 2 DIMMs
HARD DRIVE 320GB Serial ATA 3Gb/s Hard Drive (7200RPM) w/DataBurst Cache™
OPTICAL DRIVE Single Drive: 16X CD/DVD burner (DVD+/-RW) w/double layer write capability
MONITORS 20 inch UltraSharp™ 2007WFP Widescreen Digital Flat Panel
VIDEO CARD 256MB NVIDIA GeForce 8600GT-DDR3
SOUND CARD Integrated 7.1 Channel Audio
and for the HP at $1,549:
Operating System Genuine Windows Vista Home Premium (32-bit)
Processor Intel(R) Core(TM) 2 Duo processor E6700 (2.66GHz)
Memory 2GB DDR2-667MHz dual channel SDRAM (2x1024)
Graphics Card 256MB NVIDIA GeForce 8400GS, DVI-I, TV-out, HDMI
Networking 802.11 b/g Wireless LAN PCI Card
Hard Drive 320GB 7200 rpm SATA hard drive
Primary CD/DVD Drive LightScribe 16X DVD+/-R/RW SuperMulti drive
Front Productivity Ports 15-in-1 memory card reader, 3 USB, 1394, audio
Sound Card Integrated 7.1 channel sound w/front audio ports
Monitors HP 22-inch LCD Wide Flat Panel Monitor
These systems are really a wash. The Dell has a slightly better graphics card, the HP a slightly faster processor. I like that HP offers a wireless card - Dell, for some reason, only installs wireless cards on laptops. The biggies are the monitors - the Dell one is costing you $390, and the HP is $240.
You can get a monitor for cheaper than that. You can get 20" widescreen monitors starting at $170 at www.newegg.com. Or, just go to somewhere like Circuit City where you can see them in action and you'll still pay less than the ones from Dell or HP. I'd take that money and upgrade to more ram, a non-integrated sound card like the Soundblaster Audigy, or a better video card. Something to think about is that neither of those video cards support DirectX 10. MS is really pushing this, since it's Vista only, and they're putting a lot of pressure on game developers to make DX10 games that aren't backwards compatible with DirectX9.
Get as much RAM as you can...especially if you're getting Vista and not XP. The Duo chips are fine. Quad chips are out, but there are only a few programs (and no games) at this time that would take advantage of this.