Use the search function in these forums, as there are tons of threads that talk about both the Grado SR325 & the Ultrasone PRO 750.
In my opinion, don't even bother with a $20 amp. The two headphones you mentioned will sound great out of an iPod, as do many others that are known to need good amplification (e.g., my AKG K 702 sounded good out of my iPod touch--not great, but good).
If you don't want your headphones to leak sound, consider the Ultrasone (since it's closed). Other less expensive models, such as Ultrasone's HFI-580 (great for hip-hop and rock) are also highly recommended, as is the Audio-Technica ATH-M50 (decent all-rounder).
If you decide you want a Grado, you might want to check out the SR225 instead, since it's more-often praised than the SR325, and is seen as one of Grado's best bang-for-the-buck models, along with the SR60.
Don't feel like you HAVE to spend $300. There are many headphones that cost way more than that which are not even liked very much (see ATH-W5000, for example), and there are headphones that cost much less than $300 that you will find very enjoyable. The key thing to do is to just search around the forums and read up on others' impressions, and the key things to note are that everybody's ears are different (what one hears and finds enjoyable, you might not) and that manufacturing variances can cause identical models to sound different. Basically, you're not going to feel confident that you're getting the best headphone for your needs and wants with just a day of research, so spend some time here and take it slow; enjoy the journey while you're at it, because it really is half the fun.
Good luck!
