My experience with laptops and IDE's is that the persistent storage is a bottleneck, as is single core for a developer.
An IDE uses heavily I/O to load and save source ode files. An IDE furthermore is constantly compiling code and updates syntax trees. Therefore, a dual core CPU is a must. For persistent storage, you cannot have the best available. If you are on a budget, get those OCZ Core v3's, they do 155MB/sec read and 100MB/sec write, and are €99. If you want more performance, replace your optical drive with another SSD or get 1-2 Intel X-25M. Once you go SSD, HDD seems like tech we used years ago, such a difference you will notice in usability. Compile&Run never went so fast in your IDE.
When working with VM's a lot, such as Javascript, VMware/Virtual PC, and/or having the habit to keep lotsa tools/programs running, buy the biggest amount of RAM available. Its cheap, and it can only do good.
It is not unthinkable that newer IDE's such as the upcoming VS2010 use your graphics card to accelerate graphics and other tasks using the massively parrallel amount of pipelines available in a GPU. It might be quite well that you are using it within a few years as well for development of spcific modules/graphic UI acceleration. So add a decent DX 10 GPU too with about 256MB of ram.
- SSD for I/O (Core v3, Mtron mobi or Intel x-25M)
- Dual core CPU
- 4GB/8GB RAM
- recent 256MB DX10 GPU
My personal experience: SSD's let VS2008 run like a purling baby. Since I have those SSD's, I wonder how it was years back when we used hard disks ;). Very slow windows updates and F5 times where I could get coffe, flirt with the girl at the counter down the hall, taking a dump to see that its 90% done. HDD-> SSD was the best investment I made since years. The Dual core CPU is a nice second, it really helps with snappyness in multitasking and compiling. Also, when a bug of your coding made the compiled app go 100% CPU use, it won't lock up/freeze your system.
Look for my SOF question to see why I need a GPU for development of software (business software, that is) in VS2008.
One sidenote here: I use exclusively Lenovo laptops, since they are the most robust laptops in terms of physical quality. I dropped it a few times, no scratch and its still working after about 5 years. Other benefit is the businessy-look of it: dull, to the point, and matches with all suits/clothing. You dont want to attend a business meeting with a pink/red/blue laptop.