I suspect the D-Link router you're talking about is the DGL-4300 that we reviewed.
The review shows that while it operates well in Super G mode, its performance in a mixed-mode environment (with 802.11b and g clients) was less than stellar, indicating that if it is compatible with 802.11n, its performance may not be as fast as you'd like.
More generally, nothing that is currently on the market (whether it's called MIMO, pre-N, or Super G) is guaranteed to work with equipment based on the ratified 802.11n specification. It's widely accepted that the 802.11n spec will be based on the concepts underlying MIMO technology, but the specifics have not yet been hashed out and probably won't be until mid-2006. If you can afford to upgrade both now and again in a year and your current setup is a drag on gameplay, by all means, go for the gaming router.
Otherwise, it's probably a good idea to sit tight and wait to see what becomes available once the 802.11n spec is approved. Keep in mind, too, that current 108Mbps and pre-N products are not based on industry-wide standards and that various vendors are using proprietary solutions that aren't fully compatible to implement MIMO technology. Our tests have shown that mixing equipment from different vendors can negate the speed and range enhancements gained from so-called pre-N gear.
For more information about MIMO and 802.11n, check out Allen Fear's summary of the future of Wi-Fi