depends what model and spec your dell tower is, older models use different mounting points for the motherboard, more than likely the PSU will be very low powered (240-300watts) unit, the power on button/switch may only work with dell motherboard as the connectors maybe proprietary.
if the CD/DVD drive is SATA you can reuse that, IF the power connector isn't a 2 wire dell only one that is, ram maybe reused if its DDR3, DDR4 is fairly recent so guess it won't be that, harddrive should be able to use, again depends if it's SATA or not - https://en.wikipedia.org/wiki/Serial_ATA
might be best to start from scratch as it will be easier in the long run and keep the dell for a backup pc.
what do you use a pc for, gaming, movie editing or just browsing the internet ?
what sort of budget have you got in mind ?