XPS 420 Dual Monitor Setup with ati hd4870 - cannot achieve resolution over 1280x800 on DELL 30 inch monitor

Hi,

I have 2 systems (office and home) and have been happily running an XP OS, dual monitor configuration at the office with a 30 and 24 inch monitor using a HD3470 (I believe).  The 30 inch runs native res of 2560x1600 and the 24 runs 1920x1200.

purchased an xps420 from Dell 09/05/2008 with Vista (home premium or whatever), then recently decided to upgrade one of the 24 inch monitors to a 30 inch and picked up an ATI HD4870 to drive the high res of 2560x1600 (and I upgraded the Power Supply to 700W). 

Nothing over 1280x800 worked.  I tried setting higher resolutions without any success.  lots of flickering vertical bars with some horizontal fat bars (not a black screen but might as well have been).

Since I had 2-30 inch monitors (one is wfp 3007, the other is a 3008) I swapped them, and both work fine at the office.  So, I swapped the video cards, and surprisingly, both video cards work fine at the office. I also swapped DVI cables to no avail. So, I figured it was probably Vista after seeing the frequent issues with Vista and ATI Video issues posted here and elsewhere. 

I downgraded the system to XP Pro, and was sure I had the problem solved, but no luck.  I still cannot get the resolution on the 30inch screen higher than 1280x800 (same flickering problem), although the 2nd monitor (24 inch) happily displays at its native resolution of 1920x1200. I've tried multiple versions of ati drivers without success, by the way.

I've messed with about everything I'm comfortable with, though I haven't flashed the bios or messed much with the raid configs...  should I go there? I have no idea what to do anymore... just completely flummoxed and out of options. From what I can tell, the XPS 420 outright refuses to comply with high resolution video output for a 30 inch monitor in a dual config - seems wonky but what else can I conclude?

Work is stacking up since I figured it would be a quick fix but it has been nothing but, so yes, this is my first shot at requesting help - if unsuccessful I'll have to call Dell in hopes they can provide a solution.  I've also posted a very similar message in ati forums, though I suspect they'll point to Dell anyway since the same issue arose on 2 distinctly different ATI cards.  the only areas left kind of lead me into bios/sata raid issues that I'm apprehensive about going down as it is getting beyond my comfort zone

for what its worth, I grabbed SIW.exe and here's some specs - useful or not well, not really sure...  any help/advice would be appreciated.

Number of CPU(s) One Physical Processor / 4 Cores / 4 Logical Processors / 64 bits
CPU Full Name     Intel(R) Core(TM)2 Quad CPU Q9300 @ 2.50GHz
Vendor    GenuineIntel
CPU Name     Intel Core 2 Quad Q9300
CPU Code Name     Yorkfield
Platform Name     Socket 775 LGA
Revision     M1
Technology     45 nm
Instructions     MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, ET64, XD, VMX, SMX, EST
Original Clock     2500 MHz
Original System Clock    333 MHz
Original Multiplier    7.5
CPU Clock     2496 MHz
System Clock     332.5 MHz
FSB         1330.1 MHz
L1 Data Cache     4 x 32 KBytes
L1 Instr Cache     4 x 32 KBytes
L2 Cache     2 x 3072 KBytes
DRAM FREQ     399.0 MHz
North Bridge    Intel X38 Revision 00
South Bridge    Intel 82801IR (ICH9R) Revision 00
CPU    Intel(R) Core(TM)2 Quad CPU Q9300 @ 2.50GHz
Cpu Socket    Socket 775 LGA
   
System Slots    6 PCI
Memory Summary   
Maximum Capacity    8192 MBytes
Memory Slots    4

actual RAM: 4GB

thanks in advance,

D