i prefer to launch my HDL-games from the browser directly, but as i understand it OPL gives you more options like virtual memory card and gsm (videomode tweaks), so i installed it on the hdd as well.
to transfer the games to the ps2 hdd over network, using HDLGameInstaller.elf (PS2) and HDLGManClient.exe (PC) works like a charm (and the games work without fixing on HDDOSD), attaching the hdd locally to the computer gives you faster transfer speeds, but when using WinHIIP you have to fix the games with HDLGameInstaller.elf (if you want to launch them on HDDOSD).
in a nutshell:
1. dump the game on the computer = iso
2. transfer the iso to the ps2 hdd either locally attached to the computer or over network = hdl-game install
3. launch your hdl-game install either directly from the browser (if HDDOSD) or with OPL
hope this helps you with modding your phat ps2.
i would also recommend to use a internal hdd with the original sony network adapter. also recommended is to mod your original sony network adapter from ide to sata. this way i was able to install a sata ssd 120gb hdd without any hassle. game loading times are really fast and cutscenes work perfectly, even dual-layer DVD9 games like God of War install/run fine.