Sound card upgrade. Need advice

What's your budget?

Good results can be had with low cost hardware, but you might want something high end?

In terms of low latency it's not expensive to get a low latency solution for Pianoteq. A low cost entry level sound interface with ASIO running in exclusive mode should do that. 

Do you require the ability to record external instruments such as real guitars, hardware synthesisers and human voice?

Laptop or desktop?

Dedicated audio chipsets are less and less of an issue as modern CPUs can handle most of the additional load.  While ASIO4ALL isn't perfect by any means, there are very few dedicated production-grade sound cards new on the market.  If you absolutely need a better interface there are old and new Creative brand and EVGA brand options that are very good--albeit expensive (and more designed for amplification than actual sound processing like the cards of the early aughts).  There are also ways in Windows 10 of still getting older professional-grade cards to work from the used market.  Everything else on the market is overkill unless you're planning on doing XLR-recording or need other specialized features that don't directly relate to Pianoteq.  As a general rule, upgrading speakers or changing your audio configuration or drivers you're using will be a better place to start (and potentially a better value)--as onboard motherboard sound cards are basically as good as anything that you can by as an external device or internal card.  Same with USB-based audio interfaces for laptops.