Weird PC Trouble

I have the following configuration:-
CPU-AMD Opteron 175(Dual Core) @ 2.21Ghz
Motherboard-Asus A8NE-Nvidia Nforce4 Ultra Chipset(socket 939)
Ram-3Gb--1GB(512x2)Transcend ,2GB(1GBx2)Corsair value-select ram
XFX geforce 6600gt—256mb
200Gb hard drive-120gb Seagate(master)+80 gb Samsung(Slave)
400watts generic powersupply

I have the following questions:-
(1) I am currently running windows xp professional with sp2,but I did load windows vista beta –the final release version as well as windows vista ultimate edition –and I tried overclocking my cpu–on overclocking the cpu from the bios ,I couldn’t boot into windows vista ultimate or the beta version.it would get to the loading screen and then the pc would restart itself-the changes in the cpu clock frequencies would show up in the bios but I couldn’t load windows vista.does anyone face this problem? Also on using asus AiBooster the overclocking tool in vista doesn’t seem to show any change in the cpu speed . however I have no issues with overclocking my cpu through the bios and running windows xp. Can anyone help me?

(2) On changing my cpu from the AMD 64 Athlon 3000+ to the dual core opteron has given a huge boost in gaming performance—frame rates have gone up considerably but I face this weird problem while playing games— in games like call of duty 1 and 2 as well as the expansion pack, certain things which are scripted and essential for the mission to proceed forward don’t happen. For example in call of duty 1 ,in the second mission where you are supposed to get back to your hq in a French car through the german lies along with pvt.elder and sgt.moody , there is this place where a tiger tank is supposed to show up and blow the house you and your team are supposed to get into,the tank doesn’t show up!! I can’t proceed with the game.the game doesn’t crash but I am sitting in the car and waiting for the house to blow up.this never happened with my old cpu.the games have been patched with the latest patches. i can’t proceed similarly in call of duty 2 because some event that is scripted to happen doesn’t happen! I have no problems in half life 2,medal of honor(even though its an old game) doom 3,far cry,nfs most wanted/ carbon.i am trying to play as many games as I can to see if this problem is isolated to call of duty only or if it affects other games too.

(3) I need to buy a new graphics card but I can’t decide on what to buy. my 6600 gt has served me wonderfully and it was worth owning it. I didn’t upgrade to the 7600gt because there wasn’t much difference in the performance when compared to the 6600gt. tomshardware reviews and anandtech’s reviews about the 8600gt in gaming tests show that there isn’t much difference between the 8600gt and the 7600gt.the 8800gts -320 mb is out of my budget .i play games on my 17 inches Philips crt . i will buy a viewsonic 19 inces widescreen lcd monitor soon .to play games with the eye candy set to maximum will a 8600gt card suffice? How about an ati variant? Like the 2600 ? I know tomshardware had an article about playing games on using video cards having an nvidia chip on a motherboard with an ATI chipset and vice versa –that it doesn’t effect the final performance—but recently a friend bought this Sapphire ATI x1950 pro 512mb card and he has an asus A8n-v-csm –nforce4 chipset based motherboard—the sapphire card didn’t work-the monitor remained blank,he exchanged it for a Leadtek 7900gs card—which worked perfectly. I have never used an ATI card before so I am a bit skeptical.

(4)i need to upgrade my powersupply –will a 500watts psu be good enough?? I am going to over clock my cpu/ram/graphics card.the 400 watts generic psu isn’t good enough.antec would be a good choice?? Also there are different variants in antec psus.which one do I buy? The true power or neo power or some fanless variant?? I need a good solid psu.

Thanks in advance!!

I've heard good stuff about the Hiper 530w with heavy duty cable select setup. It's on sale for $69.99 at newegg. For the video card, I wouldn't get anything less than the 320 meg version of the 8800gts. I would wait until you have enough money for it.

Well, the 8600GT is a good card, but at resolutions higher than 1280x1024 it's going to have a hard time with high detail settings. It would certainly be one HELL of an upgrade from your 6600GT! I would really recommend saving some money up or cutting back on a few other components and getting the 8800GTS 320 if you feel the need for DX10. I've seen them for around $250, $100 more than the 8600GT, but more than twice the performance. If that can't be done, DX10 is still far from required right now. DX9 will be around for a while and the cards I mentioned before will perform better at higher resolutions than the 8600GT.