Er, what is a mid-range price in the funny money you guys use? :P
Something like...
Mobo: A P35 chipset board like the ASUS P5K would be cheapest and do everything you need, they also overclock really well and there are fancier versions like the deluxe/wifi if you fancy something more flashy or want better overclocking/more SATA ports or something. If you are thinking about OC'ing (and the Core 2's obviously OC tremendously well) then something with a more advanced VRM would be better... usually the black PCB versions are at least 8 phase for ASUS. There are also the P5Q series based on the P45 chipset, which just has a bunch more PCI-Express lanes... but that makes bugger all difference unless you're kitting out a multi-GPU rig, so just grab whatever variant you like the look of tbh.. there are loads.
Intel is the only game in town chipset-wise. nVidia's attempts are consistently dire.
CPU: Basically despite the imminent release of Nehalem in the next few days, the current Core 2 series is still the best way to go. The E8200 is 2.66GHz to the 8400's 3GHz and the newegg price difference is $7, both have the same cache etc so tbh the E8400 is the best value of all the 8x00 models... haven't seen one yet that hasn't hit 3.6GHz even just on the stock cooling. They will potentially go much higher too.
Q9300/9400s are down on cache compared to the Q9450 and upwards, but the benchies show its not as big a handicap as you may expect and given the price grabbing a q9300 and ramming it up into the 3GHz range gives you really good value. Still, the duallie 8x00s have higher top speed potential and
clockspeed is still king when it comes to performance. The yorkfields really are poor for desktop users because, honestly, there is bugger all that can make any use of all four cores.
RAM: These are all on a 1066 bus, but tbh just grab the cheaper DDR2 800 and whack the FSB up to 400 and there are bugger all bandwidth issues.... everything is bottlenecked by the MCH (which nehalem is moving ondie same as AMD did back in 2003), so just bung it up to 400 and don't worry about it. Faster DDR2 might be better if you're going over the 400FSB mark and you want to keep the RAM at 1:1, but tbh there is very little performance drop in using a divider these days so its not a big deal. The P35 onwards doesn't suffer from the memory strap issue the 965P had so no worries there.
So yeah, if you can save some $$$ buy DDR2800, or 1066 if its only pennies difference. Corsair Value Select (or the XMS stuff is cheap these days too), crucial/micron, adata are all good.... too much potential trouble bothering with anyone else tbh.
Don't worry about latency, its largely irrelevant.
Graphics: Well the 4850s are kings at the moment in value terms. The 9800GTs are equivalently priced but they're just rebranded 8800GTs (well, there are minor differences) and can't quite keep up. The revised GTX260+'s are great competition for the 4870 but they probably wouldn't fit into a modest budget.
Going AMD over nVidia gains you DX10.1 support (which even the GTX series lacks), but loses you PhysX support. Due to Vista's WDDM requiring driver homegeneity you won't be able to pop a cheap geffer in with a Raddy for physics unless you're running XP... so thats one less option too.
PSU: anything Seasonic 400W range, Corsair VX series, HX520 if the budget is there for it although its total overkill.
HDD: Samsung Spinpoint F1s are the business for most people. Good alternative is the WD6400AAKS which are half the price and absolutely stonkingly quick, not far behind the F1 and way ahead of all it's 250GB platter rivals.
Gimme a rough budget to work to and I'll give newegg a once over and knock a quote up
There's also more blathering on component options
here... its relatively up to date.
[Edit: Nehalem NDAs have
expired ]