I'm not a big Dell fan, but if you really don't want to build your own computer, here is a high-quality, high-performance workstation with lots of expansion capabilities --
Dell Workstation 530 --
Intel Xeon Processor, 2.0 ghz, 512k cache
512 mb PC800 ECC RDRAM, 2 RIMMs
Enhanced Performance USB Keyboard for Windows XP
Microsoft USB Intellimouse
3 years parts and onsite labor
ATI Radeon VE, 32 mb (dual monitor capable)
Microsoft Windows XP Professional
Dual 18 gb Ultra 160 SCSI hard drives (10,000 rpm)
16X DVD-ROM for Radeon VE and Windows XP
As configured, this system is $2859, but Dell is currently offering $200 off a $2700+ system. That brings the price to $2659. I frequently get mailers from Dell with coupon codes for 10% off any system. That brings the price below your $2500 target. Of course you still need a monitor and whatever software you want.
As mentioned, this system has great expansion capabilities. The motherboard supports dual processors, so you could add a second Xeon CPU later. The motherboard has 8 RIMM slots! Plenty of room for more memory. Sound and networking are integrated on the motherboard, so you will want to add a sound card if you need high-quality sound.
As configured, this system will be extremely fast. If you add a second CPU, more memory, and more Ultra 160 drives, you will have a system that will command the respect of even die hard Athlon fans.
By the way, I skimped on the video card to get the system in your price range. If 3D performance is important to you, you will want to upgrade to something like a Quadro2 Pro, 64 mb, for an additional $360.
Also be aware that you could build a DDR-based system around an Athlon XP or Pentium 4 Northwood and get comparable performance for less money.