I'm building a computer (pc)  for my husband and father in law. my fater in law is very impatiant so my husband thinks we should get a quad core. Will that make a noticable difference vs a dual core?  (all other hardware being equal)  The computer will mostly be used for checking markets, email, shopping on line, and watching dvds. The computer will probably have win xp. Also will that make a differenence? Or do we need vista to use a quad core?

Thanks

For the applications you mention, extra cores won't make any difference. For watching DVDs you don't need any extra speed and for anything online the internet connection will be the bottleneck. If you really want to spend the extra money, get a faster hard drive instead.

If he intends to use Poser 7 however, then yes, the quad core will help rendering faster.

For the applications you mention an entry level PC will be more than sufficient.
Faster hard drive - a current generation 7200 RPM /32 MB cache SATA II drive will be fast enough. A Velociraptor might be slightly faster - for this kind of applications - but the difference will be minimal and probably imperceptible. Shelling out the extra money for a Velociraptor is a waste.

I'd recommend an entry level Core 2 Duo PC with 2 GB of RAM (2 x 1 GB memory module), with a decent ATI graphics card - at this time, ATI does better with plain 2D video than nVidia. No need to buy an expensive gamer card, a HD 3650/4650 is good enough.
No integrated graphics, they're slow.
In the ATI range, the first digit indicates the generation (2, 3 or 4), the second digit the chipset used in the generation (higher numbers are faster, with more memory bandwidth), and the last two digits indicate memory speed.

This means that an ATI HD3650 is significantly faster than a HD 4350. Don't go for lower than a '6' for the second digit, although an '8' will be overkill.
I woudn't recommend a 4850 or 4870, those cards run far too hot to my taste. The graphics cards will hold, they're designed to run at high temperature, but they will heat up the mainboard that is NOT designed for those temperatures.

CPU: Intel Pentium 4 E5300 is affordable and more than fast enough.
Mainboard: don't skimp here. Get a good quality, but fairly simple mainboard. Gigabyte is the recommended brand these days, according to several shops they're the most reliable boards on the consumer market. The Gigabyte EP43-DS3L is a rock solid affordable mainboard.
I wouldn't recommend a mainboard with an nVidia chipset these days. I've had several nVidia chipset failures due to heat last December - I burned my finger when I touched the heatsink - while an Intel chipset under the same or heavier loads only got lukewarm. The Gigabyte EP43-DS3L has an Intel P43 chipset,. Not the fastest on the planet, but reliable and doesn't produce that much heat.

Hard disk: Samsung Spinpoint F1 series. Take the 750 GB or 1 TB variety, those have a 32 MB cache and are faster than the smaller versions.
Fast, reliable, silent and affordable. Can't go wrong there.

PSU: get a good silent PSU with stable voltage output and cable management. The Coolermaster M520 gets great reviews, it's silent, reliable, has stable outputs, cable managent, and is still affordable. 520 W is more than enough to power that machine - it could easily drive a quad with 4 disks and a fast 3D graphics card.

System case: again, don't skimp there. A case that is both solid and silent is pretty hard to find, but a good system case will hold several generations of innards (I've got an 8 year old Chieftec case that is holding its fourth mainboard right now. Rock solid, but alas, not silent).
Antec makes silent cases that are fairly solid, but the price can be hefty.

Thanks. that might help. No, no posering on that computer. One of the users can only point and click. if there isn't an icon on the desktop, he can't figure it out. The other only uses it for work.