My Card : GeForce 4 Ti4200 64MB DDR (SUMA-Platinum SE)

- The Video Memory show exactly 64MB after clean install Win XP Pro + SP1 + DX8.1 or 9.0 + Every Drivers install & update everything at windows update.

- I check the video memory everytime i start the machine (by right click on desktop 'Properties' --> 'Settings' --> 'Advance' --> 'GeForce 4 Ti4200')
its show 64MB exactly, so this is not a hardware problem,ok.
(first time i use Det.41.02-beta , 2nd times 41.09-official for my WinXP fresh install)

- Once i update my video card drivers (many version i have tried, official or unofficial , whql or non whql.
the video memory start freaking around, sometimes its show 16mb, sometimes 32mb, sometimes 64, but its never stick to 64mb.

- I have change my mother 2 times : Asus A7V333 (VIA-KT333) & now Asus A7N8X (nVIDIA nForce2) same problem happen.
I remember before nvidia Detonator drivers 40.xx , WinXP SP1 & DX 9.0 come out (that times i'm using my A7V333 MB) i don't facing this weird problem at all , even i try install many different nvidia driver.

- The Question is : it is the nVIDIA drivers problem? or WinXP SP1 problem or DX 8.1/9.0 problem (i have seen a post an ATI user having the same problem, video memory change randomly.)

my setup :
AMD Athlon +1800 (1533mhz) (T'Bred rev.B)
ASUS A7N8X nForce2 Mainboard (non Deluxe / BIOS 1002 Final / Drivers ASUS 1.16 Official)
KingMax PC2700 512MB DDR (Run@333mhz) (@DDR Slot 2,3)
SUMA Platinum SE GeForce 4 Ti 4200 64MB (Default Memory clock 550mhz@3.3ns)
HighPoint Rocket RAID 404 (@PCI 5 / Drivers v1.23)
2x Maxtor 80GB 7200 ATA133 (512K RAID 0)
2x Maxtor 40GB 7200 ATA133 (16K RAID 0)
Creative Audigy (@PCI 2 / Drivers v5.12.1.283)
U.S.Robotics NIC (@PCI 1)
WinFast TV 2000 XP Deluxe (@PCI 3)
Yamaha CDRW 24X & ASUS DVDROM 16X (@Primary IDE 1)
Yamaha CDRW F1 44X & ASUS CDROM 52X (@Secondary IDE 2)
V-Tech 350Watt PSU (for Mainboard, HDD, CPU fan)
Enlight 300Watt PSU (for Optical Device , System fan)
4 On-Board USB port are in used (USB 1.1/2.0 Enable in BIOS)

lets see... did u call PNY or return the card???

Like i said, it is not a hardware problem, because if i do a clean install of XP and stick with the 1st nvidia drivers i installed (i try 41.03-beta and 41.09-official) this will not gonna happen. The problem only happen when i update to another version of nvidia drivers, no matter beta or official.

I had this same problem and it had been driving me crazy until a few moments ago
In my case, I had inadvertently changed a performance setting that I knew instinctively should be the other way, especially since I'm a gamer. Ready for this? I had accidently somehow, someway changed Memory Usage from "Programs" to "System Cache". I know better. It should be "Programs". Apparently for XP, this had an adverse or unstable affect on the video memory or at least how it was recognized by windows. Since I never thought those performace settings would affect the video memory, I hadn't bother to delve all the way down to "System Properties\Advanced\Performance\Advanced" to find that setting was incorrect. I only stumbled across it because a friend had suggested I should turn "Write Combining" off. While trying to remember where "Write Combining" is and searching for it, I called up those performance settings and saw that incorrect setting and fixed it. Then without rebooting, I found "Write Combining" and turn it off. I rebooted and I had 128MB of video memory! But, I realized I had made two changes and I had to know whether one or the other caused it or if it took both. So, I systematically, reversed the changes and found that only the "System Cache-to-Programs" change affected the video memory.
I have to figure either people that use the "System Cache" setting either don't realize the adverse affect or Microsoft is not fully aware of what this setting does (i.e., maybe it's a WinXP bug). Obviously, Microsoft doesn't warn through Windows Help how this affects video memory. It's almost like XP is borrowing video memory to increase the system cache.