First, allow me to start with my standard statement: Macs are evil. For proof, i present to you the fact that clstr 7 is a mac. I rest my case.
$3k is a very respectable laptop budget. For that price you can get into the current state of the industry Alienware laptop. Core2 duo processor, SLi graphics, Raid 0 array. In a laptop. http://www.alienware.com/product_detail_pages/area-51_m9750/area-51m_overview.aspx?SysCode=PC-LT-AREA51M9750&SubCode=SKU-DEFAULT
The downside is the thing is heavy. VERY heavy. And it has multiple exhaust ports on the bottom so you would only use it ona table, never on a lap. Otherwise you will cook something. That one also lacks a docking port, which means you would have to plug in and unplug cables whenever you wanted to go mobile.
If you don't want to go that large, but you still want a workhorse you could got with a dell laptop. Gateway makes some decent models in the tablet / convertable model range but most of their desktops lag behind dell in terms of technology. Thinkpad got bought out by china and became crap. SOme of HP's higher end models are actually decent this year, but I don't trust their notebooks since they integrated compaq.
Id go with this if I wanted docking port capability and about the same performance as the alienware, but 5 pounds lighter: http://www.dell.com/content/products/productdetails.aspx/xpsnb_m1710?c=us&cs=19&l=en&s=dhs
If you wanted one a little less flashy but basically indestructable, look into the Dell Latitude buisiness class machine line. The D830 just got released, but the graphics options for it sucked last I checked because they were waiting on the 8800 mobile chips. But the D820 is an excellent laptop, lightweight, and very well built. We haven't had any part failures on our fleet, and we have had them for around 4 months now.
You could try and build a "whitebox" laptop, but the parts just aren't cheap enough or flexable enough to make it worth the time.