Problems with 6800GS; need general GPU advice

hi, i'm new to the forums - i used the Short List sticky recently as a guide when looking for a reasonably priced AGP video card to hold me over until i build a new, PCI-e-based system sometime in the middle of next year. i settled on the GF6800GS, which i found (after much searching) for $169+shipping from a pricegrabber merchant. these cards are rare at this point, and i couldn't find one anywhere else for less than ~$240. unfortunately, when i received the card and plugged it in, it produced significant artifacts in F.E.A.R. and HL2:Ep1. i tried both the drivers that came with the card and the most recent ForceWare drivers from nVidia's site, but both produced the same results. interestingly enough, i downloaded one of the 6800 series graphical demos from nVidia (the boat demo), and it ran without a hitch.

at the price i found, the 6800GS seems like pretty much a slam dunk, with OpenEXR HDR compatibility and pretty good speed for a mid-range AGP card. naturally, i'm a little reluctant to send it back, because the merchant i bought it from has since sold out of the card, and i doubt i'll be able to find it anywhere else for a reasonable price. thus, i have a couple of questions.

1. is there any other explanation for the card's producing artifacts right out of the box, aside from it's being faulty? does an underpowered GPU tend to produce artifacts? i never had problems with my previous card, a 9800pro, but i assume the 6800GS is significantly more power-hungry, and my 420W power supply is already feeding a DVD+/-RW and four HDDs.

2. what could be causing the card to produce artifacts in the two games i mentioned, but not in the nVidia demo? the demo should presumably be giving the card a pretty good workout, because its description touts some relatively advanced shader techniques and such.

3. if the card is clearly faulty, what's my next best option? i had settled on the 6800GS over the x800GTO in large part because i really like the HDR effect and wanted to experience it in games such as Oblivion and FarCry, which require FP16 blending. i briefly considered an x1600pro, since it supports HDR (and simultaneous AA), but have read that these cards are really underperformers and wouldn't keep up with either of the former two cards. i suppose a good followup question would be, is either the 6800GS or the x800GTO capable of running Oblivion at a reasonable framerate in the first place?

4. given my situation (wanting a temporary upgrade to hold me over for ~1 year, until i upgrade my entire system), would it be wise to settle for an x800GTO, or to grab a cheap PCI-e motherboard (such as an Asus A8N5X, <$80 from Newegg) and a 7600GT for slightly more? it bothers me that i could get more for my money if i were buying a PCI-e video card, but at the same time, a motherboard upgrade as a temporary fix seems like a bit of overkill.

any answers/advice that anyone can give would be quite welcome. thanks in advance  

EDIT: i can post some screenshots of the artifacts i observed this evening, if necessary.