$2500. That can easily land you the best parts on the market. In fact, $1500 USD would be able to. Here's a PC parts buyer's guide to demonstrate:
It doesn't mention the GTX 580 which is $500 USD and greatly overkill unless you have a huge monitor, but that's still easily within your budget.
Some words of advice though. Intel Sandy Bridge undoubtedly offers the best performance (even beating a 6-core Core i7-980X in some tests), but the motherboards have a design flaw involving the SATAII controller (20-30% of being affected, which will cause the I/O to degrade in 1-3 years rendering them inoperable). SATA III ports are unaffected though, and you can use a PCI-E port to add extra SATA II/III ports as necessary. If you follow Raven's advice and get a SSD and 1TB HDD (along with a DVD drive), you have to keep that in mind as there are only 2 "safe" ports. Revised motherboards with B3-stepping (instead of the faulty B2) are expected in the coming couple of months.
Also, with SSDs, the SandForce 2000 controller is showing up on the market, and the OCZ Vertex 3 (being released in around 3 months) has already shown speeds that completely dominate the current genreation of SSDs. In fact, it's arguably the only SSD to ever show the full potential of a 6Gbs SATA III port - and only Sandy Bridge motherboards can get that (PCI-E based SATAIII controllers have limited bandwidth due to PCI-E design, and non-Sandy Bridge motherboards featuring SATA III use a single PCI-E lane to power it so they have the same restriction). Either way, both the current generation of SSDs and the generation that's just around the corner are much, much faster than HDDs. Also, if you have to get a current generation SSD, the Crucial C300 series are the best on the market.
The only other part of the system to consider is the graphics, but Rydian and Raven have that covered. Consider your needs, consider your monitor, consider your budget. The mid-ranged GTX 560 is plenty powerful for any game at maxed settings and overkill if you've only got a 19" monitor. If you've got a 24" or bigger monitor, you might consider a high-end graphics card like the HD6970 or GTX 570 so you can pump up the AA and AF. Also consider if you want more than one monitor - the HD6970 is ideal for EyeFinite.
EDIT: A 70 year old lady I know can build a computer with YouTube videos. It's relatively easy anyway - open the case, put the back-plate on (comes with the motherboard), ease the motherboard into position, screw it down. Put the power supply in, keep the cables out of the way for now, connect any case fans to the motherboard, then start putting in the HDDs and DVD drive. Connect the front panel connectors to the bottom right of the motherboard, start plugging in the power cables, plug in the SATA cables, plug in the graphics card, plug in the power cables for the graphics card, and use the holes/space in the case to tie up any loose cables. Close motherboard, connect power, and monitor cables, connect keyboard and mouse, boot up Windows/Linux installation CD. Follow the on-screen instructions. Did I forget anything?