Does the 40K include the interface cost? Those things get really expensive really quick. What about monitoring? Is that to be included as well? A pair of decent monitors will set you back by about 40K all by themselves.
If the cost is only for the box and bits, a desktop makes much more sense unless you have to do live sound as well, that is when a Macbook Pro will help. I'm not really sure you get a Pro model for anything less than 60K but at 40K it would be a good bet.
In any case if you do decide to go Wintel, be aware that Windows is terrible for music production. You can make XP work for you somewhat, but Windows 7 has high enough DPC latency to severely limit your recording ability. So it's either crashes galore on XP, or messed up overdubs on Windows 7, you choose. Macs will definitely be better, but be prepared to spend over 60K on a Mac with sufficient processing power.
In a Wintel box the thing is to go with as much memory as possible, and as far as possible stay with Intel, including Intel original boards. Their sucky performance is compensated by guaranteed compatibility with software and hardware, which in a studio situation becomes the most important thing - you wouldn't want three-hour recording sessions interrupted by a crash. Ensure you have a quiet system - good fans and a nice quiet case will cost a bit more but will help during mixdown and recording. A quiet cooler and a fast dual-core processor is the way to go, render speed is usually more important than parallelism unless you run 40+ tracks with lots of DSP.
I'm sure people here will fill in the blanks with exactly what to buy, and you'll get a lot more suggestions
When I used to produce I was on AMD (I still am exclusively on AMD) and it wasn't too bad, but there were always a few niggles with hardware drivers and latency. Those have since seemed to sort themselves out, but for the short period of time I used an Intel original board production was a snap, with extremely low latency, nice clean power to PCI boards and a reasonably quieter system at default than AMD. I'm sure nothing much has changed since then, at least not in the parameters that matter for this functionality.