Crystal Upgrades
As one might expect these cards are very cheap but alongside that fact they are also very easy to modify, with the basic de-soldering and soldering of new crystals higher sampling rates can be achieved.
The "new cards" using PCIe have easy to work with 2 leg SMD (Surface Mount Device) HC-49/US type crystal packages this abundant cheap and available worldwide.
For Reference the HC-49/US type looks like this:
Below is a list verified and tested by Tony Anderson and others who has been working on modified cards for a while alongside utilities and driver modifications, credit goes to him for the information generated for modifications.
Native SMD crystal on current PCIe 1x cards is a HC-49/US type
40MHz - ABRACON ABLS2-40.000MHZ-D4YF-T 18pF 30ppm HC-49/US (Found by Harry Munday)
40MHz - Diodes Incorporated FL400WFQA1 7pF 10ppm SMD Package
48MHz - ECS ECS-480-18-33-AGM-TR 18pF 25ppm SMD Package
50MHz - ECS ECS-500-18-33-AGM-TR 18pF 30ppm SMD Package
54MHz - ECS ECS-540-8-33-JTN-TR 8pF 20ppm SMD Package
Upto 57mhz has been tested possible with the chipset but 40mhz has been achievable by all tested chips with others not able to take 54mhz the limit expressed on the data sheets.
Using a Pi Pico you can software define 28/40/50/54 and higher but requires a restart each time, this method also allows for multiple cards to be synchronised together with external hardware such as an audio PCM1802 ADC for capturing standard baseband audio signals.