Depends. You will need to figure out if the sound bar is passive or active.
Passive sound bars include one or several speakers for each of the front channels (i.e., left, center and right). In the back of the speaker is a connection for each channel, which you need to attach to an amplifier or receiver. Simply because they have no built-in power amps. So, while passive speakers may be cheaper than active sound bars, they require a surround receiver to even produce front-channel results.
Active sound bars have the same configuration as passive sound bars, but they include built-in amplifiers and surround sound processors to separate the center channel sound. This makes active sound bars more expensive than passive sound bars, but the advantage is that you don't need a bulky external amplifier to produce basic front channel sounds.
Active sound bars should be connected directly to your TV and/or sources. Passive sound bars can be used to replace the front 3 speakers in a HT setup.
A sound bar is designed to create the illusion of surround sound. It is not an enhanced center channel speaker. If you try to combine it with other speakers, you will only skew the sound quality - especially if you try to pair it with a different brand of speakers.
Since you were planning to purchase a receiver and some additional speakers. I would just buy a matching set and get rid of the old 2.1 system it will be cheaper and sound better.
Now your options will all depend on what your budget is or what you can afford. I am privy to Pioneer A/V Receivers and Polk discrete speakers. Everyone will tell you their system is better than others. It is all subjective. All you can do is research and try them out with your ears. Bring your own audio too, as most outfits will have something more enhanced than usual to make the sound better to cater the sell.
Practically, all speaker manufacturers make some 5.1 speaker package set. Look at these links, they will get you started. I am sure there are a myriad of other sites too.
https
/www.soundandvision.com/
http
/hometheaterreview.com/