... is fairly typical of a heavy-duty machine used in the financial sector. A Dell Precision 490 with dual dual-core Xeon 3Ghz processors, 4Gb of RAM, 32-bit Windows XP and a dedicated PCI-E SAS controller with two 15Krpm SAS drives in RAID1, and I use two 2407WFP monitors on that PC running off the NVidia Quadro FX4500 display adapter (which is overkill for this use - a Quadro NVS-series card is all you need if you're not doing 3D visualisations, etc). These are all factory options if you go to Dell.com, although the FX4500 has been superceded.
A direct HP alternative would be the xw series, with the xw8400 range being a suitable candidate. We have a few of them down at the lab, and the quality / performance / service is broadly comparable to the Dell - although the HP's might have a slight edge.
In terms of power that you'll need, even running the giant Bloomberg link-infested Excel models from the fund managers / other industry types I work with, the horsepower required is actually well within the capabilities of a low-end quad-core consumer PC. The distinction between buying a workstation or a PC for this kind of task is almost purely about what sort of resilience and support you expect. With the Precisions I get same-day onsite service, the machines are rated to run continuously at high loads in high ambient temperatures - as well as other features adding resilience in operation.
If on the other hand you're looking for a more general-purpose machine on which you can do your trading then slack off at the end of the day and have some fun, then I can recommend the Dell XPS H2C desktops, although I'm also looking to acquire a couple of HP Blackbirds as soon as it is available in the UK. I've found the H2C computers quiet, powerful and reliable under stress although not as future-proof as the ubernerds would like. Next-business day service only though, and the support follow through isn't in my experience as thorough as the Precisions.