With higher video resolutions comes a higher demand for storage space, so the 8GB Iphone may fall short as far as the amount of MP3s and videos you can store on it, this depends on the user of course. At 128kbps the average MP3 size would be of around 3.5 Megabytes, dividing the 8GB (1GB = 1024 MB) we have that the phone can roughly hold 2000 songs, this number will vary as the bit rate used to encode the songs changes, the most popular choices for MP3 encoding are 96, 128, 160, 192 and 256 kbps.
The average video size which is compressed to fit on a CD is roughly 700MB this size depends on the length of the movie and the audio/video quality which needs to be kept, the Iphone can handle resolutions of up to 640X480 in H.264 or MP4 formats meaning that you can roughly transfer 8 movies to the phone and have space for 500 songs. Another resolution supported by the Iphone is 480X320 which you can use depending on the video.
If you encode the video yourself using popular encoding applications such as Virtualdub you might want to make sure not to use any type of special video filter to improve the video quality so choosing a bicubic encoding mode will be more than enough, if you choose to add other features during the encoding process such as multiple-pass with external .SRT subtitles the video might playback but the subtitles won't since they need to be hard coded in the video in order to become visible.
Encoding video at higher resolutions would be a waste of time and processing power since the device can't handle higher resolutions, in many cases when the file is of higher resolution than the one the player can handle the file won't play back or it will simply be rejected by the device during transfer because it exceeds its capabilities.
Depending on the amount of digital media you are planning to carry around you might consider researching other alternatives, however if you don't plan on watching many movies on it the multimedia capabilities of this device will suffice.