Well i have a DFI nF4 ultra-d with g.skill and it works just fine. I think DFI has problems with corsiar. I'm not sure though but ur g.skill should work fine. Personally, i would nvr go open box, but if u want, you could try.
btw, i was lookin at ur X1900XT OC, is that the highest ur core clock can go? I have mine set at 689 but the Memory is only 750. I havent OCed the memory that much, i will soon though.
And also, on Xtremesystems, a guy tested a X1950 mastercard usin a X1900XT as the slave card and Crossfire worked. But i would probably get the 1900 mastercard since their are cheaper now.
*Opteron 165 1.8Ghz
*DFI Lanparty UT nF4 Ultra-D
*HIS Hightech X1900XT 512MB w/ Zalman VF900 (Core: 690Mhz, Mem: 1500Mhz)
*G.Skill 1GB DDR 400
*Kingwin Mutant X Series
*HP IDE Lightscribe DVD Burner
*Seagate Barracuda 80GB SATA II
*Maxtor 80GB IDE
*OCZ Gamexstream 600W PSU