[goldorak]

Originally Posted by subdude

since my trusted old pc is coming to his end i want to get a new gaming computer. i have thought about something along the lines of an intel Q9550 CPU, 4GB RAM, Nvidia GTX 280 VGA and a motherboard that takes all that. then of course the normal other stuff like a dvd reader (i don't burn), hard drive, power supply
i have chosen these brands for no other reasons than these are the same as in my existing PC.
so this is where i am going with this:

Lets start :

does a Nvidia VGA have an advantage over ATI ot vice versa?

Insofar as windows is concerned no, one is equivalent to the other. In same price-perfomance range, the performance will be within 1-4%.
Nothing you can even remotely notice in games.
Neither brand is perfect, so as you will have from time to time problems with Nvidia's drivers, you'll have them also with ATI's.
Now if you're concerned about using your graphic card in another os such as linux/freebsd/opensolaris besides windows then the only viable alternativa for the moment is an Nvidia based card if you want all the features.

does an Intel CPU have an advantage over AMD or vice versa?

Yes and no.
If you don't consider the new i7's, then amd's phenom II x4 compete quite well with intel's core2quad penryns. The real difference to consider is that to get ECC ram in intel based computers you have to get a top of the line chipset (x38 $$$), whereas with amd all cpu's support ECC and most motherboard makers give you support for this features without sucking the life $$$$$ out of you.

is the new intel i7 CPU something i should consider?

i7 is fundamentally a server oriented cpu. That means that unless specific types of workload are considered you're not going to see much performance over what a really top end quadcore penryn is going to give you. Also, i7 have less l2 cache (8 MB l2 cache) than quadcore penryns (which have 12MB l2 cache) and this can impact on game performance. In the end i7 performs as well or even a little worse in games with respect to intel quadcore penryns, but smokes pretty much everything else out the water once you consider other types of workload.

is DDR2 RAM ok or should i go for DDR3?

Depends on the type of cpu and/or chipset, amd cpu's for the moment support ddr2.
For intel (excluding i7 line which supports only ddr3), you'll have some chipsets that support ddr2 and some chipsets that support ddr3.
Not much difference between ddr2 and ddr3, the real game changer is that by going to i7 you'll triplicate the bandwidth, whereas with older chipsets/processors you'll only have dual channel available (twice the bandwidth).

can i stick to windows XP or will there be games in the future that only run on vista?

Future windows games will only run on Vista/7. DX10 is not supported and never will be on XP. For the moment most games have a DX9 path so they will work on XP, but in the future no. DX10/11 games only on Vista+. Now with opengl games the situation is a bit different. You can have opengl games that have DX10 type features that work on xp (god bless opengl ) but there is not one game that supports this for the moment. Maybe id software's next project will have dx10 type effects and being an opengl game you will be able to play it in windows xp.

what is the advantage of a sound card over the onboard sound?

Mostly its a question of quality. With a dedicated sound card you're less prone to picking up random noise from the computer. Noise that affects sound quality.

any other things i have to consider?

Buy a good quality case/good fans (quiet fans) in case you're willing from time to time to tinker/upgrade the components of your computer. Oh yes, don't go cheap on the power supply. Get a nice corsair hx 620 or something equivalent, but leave 50-60$ power supplies. If they go poof there is a great probability they will take some of your components in the graveyard with it.

thanks for your help!

No problem.