Futureproof is only really relevant when architecture changes.
Single core to Dual core would be an example, and from Dual core to native Quad core, i.e. Intel Nehalem would be another. Also consider that the legacy 'front side bus' architecture of Intel motherboards will cease to exist with the Nehalems and such motherboards will be dependant upon DDR3 memory.
Thus any processesors/motherboard/ram upgrade now, would only be suitable, if it was very cheap and at the same time, considerably better than your current components.
Having said that, buying a "Nehalem" package, would be considered proper 'future proofing.'
On the GPU front, nothing about architecture has really changed much in the last several years. The G80 from Nvidia was the closest we came; and while the GT200 is considerably more powerful, it's still built around the same technology - the same of which can be said for ATi's latest offering.
Things like "Ray Tracing" when fully implemented and functional, and/or onboard physics processors, would mostly likely be the next architectural jump. However, physics is already available through Nvidia at the present time; ATi will most likely follow suit in some form or another, and the same can be said for ray tracing(or something similar). When the latter might happen, with the kinks ironed out, is pure speculation at this point, which leads me to suggest that any top end GPU on the market currently, will survive another year or more.
Power supply units have had their base design changed as well in the past several years. The introduction of the gimmicky dual or multi rail concept has bewildered a lot of consumers, though with the introduction of more and more power hungry graphics cards, the need for high amperage single 12v rails has become apparent, and people are finally and fortunatley starting to catch on. However, this is about the only real architectural difference from power supplies that we've seen. The introduction of PCI Express lines is also of interest. On that note, expect to see power supplies coming equipped with more 6pin and 8pin express lines as standard.
For single card solutions, and some SLI/CrossFire, a 800-1000 watt, single 12v rail (80AMP) or more PSU of reputable name (such as Silverstone), would last you several years, and are very affordable.
Monitors are a bit tricky, because there is so much marketing nonsense that manufacturers use to pull the wool over people's eyes. Though, there's plenty of truth about the subject if you're willing to research it. Currently between the three main panel types, TN film, PVA/S-PVA and ISPS(listed under different names sometimes), the PVA is a common favorite amongst gamers, and lower budget graphic designers/multi-media users. Generally speaking, PVA is only found on 24" or higher size monitors. This isn't always the case, yet it's close enough. Once you reach 30" and larger, you run into the ISPS, though you pay quite a premium, and for gaming it's a negligible/marginal increase in quality. Also keep in mind that the architecture of TN film might actually change come the end of 08 or as late as mid 09. The backlighting LED and various variables have undergone quite a change in some of the manufacturer's labs. Iiyama and Belinea have been working on some pretty wild advancements. The prediction is that the new line of TN film will have better quality than PVA panels, and possibly surpass that of current ISPS; whilst still retaining a lower price point than PVA or ISPS.
Needless to say, if you just want decent contrast, colour and 'head on' (gamers always facing the screen from the front...not the side..) viewing angle(s), then a TN film would be totally suitable; leaving the question really down to what size you want. However if you do go 24" or bigger, be on the lookout for a cheap PVA panel anyways. If you find one for $50 more than a 24" TN film, get the PVA, otherwise don't stress it.
Harddisks are generally about size/capacity for most consumers, although buffer size, read/seek latency, rpms and other minor features, are something to consider. At this point, and probably for a decent while, SATA II speeds, 8-14 ms seek time, 8-16mb buffer and 100gig + capacity will be the norm and are perfectly acceptable even in a 'ultimate gaming' machine.
Nehalem will be out in several months, so save up now and put your money towards that. You will pay the 'new market price premium' obviously, yet it won't leave you with that feeling of 'oh dammit, I just bought a new component and it's already 'outdated,' what a waste of money!'
Nehalem won't be outdated for quite a long time. You will be 'future proofing.'