Dell (support.dell.com) lists no Windows 7 drivers
for the machine, either X86 or X64. There are Vista drivers, but 32 bit only.
I doubt that you'd have any trouble, though.
I'm pretty sure that your CPU is X64, but I believe that there is no such thing as a model number 1720 from Intel. The freeware CPU-Z should tell you if it supports EMT64 (64 bit) instructions.
For example: I recently went to Windows 7 on a cheap HP/Compaq CQ50-105NR. I did a clean install. Most of the drivers needed were installed by Windows 7, out of the box. (The wireless network drivers were the important ones.) Most of the rest were provided by Windows Update, including the 8200M graphics. The only thing I needed to download were the nVidia chipset drivers, which weren't needed for basic functions but cleared up a missing entry in Device Manager. As your 1720 has an Intel chipset with supported drivers at Windows Update (both 32 and 64 bit), you may not have to download anything at all from Dell.
X64 would be nice with your 4GB of RAM, but the practical difference from X86 (available RAM) would be modest.
Make sure that you have a backup (image) of your Vista installation, in the event that this free advice turns out to be poor.