ASIO Direct X still sucks on my system (1 Ghz, Audiophile 2496). If that soundcard has its own ASIO drivers, it's much better to use these. And it seems it does have ASIO support: http://www.xgfactory.com/html/comp/drivers/driv_f.htm
For this you don't need to setup ASIO DirectX or ASIO Multimedia.
In Cubase: Options -> Audio Setup -> System
Then select the ASIO driver from your XG card and you'll see the latency.
If it's possible, click ASIO Control Panel (the setup of your soundcard) and maybe you can change the DMA Buffer Size there.
I use VST 5.1/32. I'm not sure if it's all the same for VST 5.0.
I vaguely remember I had troubles with 5.0, but with 5.1 it all worked perfectly so I recommend using that version.