Hey,
The difference between the normal Asus Gtx550ti, and the Direct Cu version is the following.
The normal Asus gtx550ti is a reference card, meaning it is stock and has been made to normal specifications. It will not have fancy cooling it is just the normal version of the card. The Direct CU version is a non-reference card, meaning Asus has taken the normal design and improved it. The Direct CU as ricardois said meanse direct copper cooling. The card runs cooler than the reference, and that works to your advantage. Asus Direct Cu cards claim to also have higher quality components, as they call "SAP", or in other words Super Alloy Power. These cards usually have more power phases, which allows for better results when overclocking, and supplies the card with enough clean power. These cards also usually come with a slight overclock, so you may see a small performance increase, but nothing big. At least that is how it is with their Gtx 570 Direct Cu. If it comes with a bigger overclock you may see performance increase. Asus has different models and depending on the clock speeds performance will vary, the direct Cu versions usually come with some kind of overclock.
The key points in buying a Direct Cu card are:
1. It runs cooler, and quieter
2. It has better components
3. It overclocks better. So you can overclock the card for better performance.
Hope this clarifies things