It is definitely possible to create all of these designs under 300kb on disk. Crafting a website from a PSD while keeping bandwidth usage to a minimum is a crucial aspect of being a web developer. Providing exact instructions on how to achieve this is akin to teaching a lawyer how to practice law - it's a specialized craft that requires expertise and attention to detail. Here is a glimpse into the process:
The first design is straightforward. The gray gradient can be achieved using a 1 pixel-wide image repeated with CSS. The blue blocks can be created using a .gif image with slight transparency, while the images of the woman and logo can be JPGs. By utilizing CSS and a few strategically placed images, this design can come together seamlessly.
The second design features gradients that are mostly unidirectional. Slicing these gradients into 1px wide images and repeating them horizontally is a practical approach. The yellow bar could be composed of three separate images - one for the left part, another for the right part, and a repeating middle section. The illuminated 'Thank you' text could either be a high-quality JPG or PNG file.
The third design presents a challenge due to its numerous gradients in various directions. Consulting with the designer to simplify the gradients to a single direction (such as up-down) would be beneficial. Additionally, replacing the unique large font ('Thank you') with webfonts can enhance the design while reducing file size.
In each scenario, collaborating with the designer is essential, especially when dealing with pages rich in glows, gradients, shades, and shadows. Designing web pages requires a specific skill set that differs significantly from print or other media design. A skilled web designer understands how to streamline the development process and make it more efficient for developers.
Best of luck!
[edit] I advise against using CSS3 or browser-specific solutions for rounded corners and gradients, as compatibility with browsers like IE6 or 7 may be necessary. It's also wise to limit reliance on JavaScript for functionality.