Vinyl System Upgrade Path Recommendations

I also have a Pioneer PL-530 table. Bought it in the 70's new and haven't had the choice or finances to upgrade to something "much" better. Besides whatever treatments and cleaning you do to your records there are a couple of things you should consider about the Pioneer.....
First...have you investigated whether the tonearm support may have cracked due to age? This is a common problem for this table.
Is your current cartridge set up correctly? That is, other than using the crude overhang gauge that came with the table and other mounting parameters. When I first bought the table I used the supplied AT cartridge and was satisfied with the performance for a long time. I have tried a few different cartridges over the years with different results. Whatever cartridge you decide on is your choice. Tryouts for those is mostly limited.
One thing to remember if you buy a new cartridge is to set the correct tracking weight in accordance to the specified value.....This is a direct drive turntable. The most sensible thing to do is to set all the weight values to zero, set the tonearm so it is level, and then dial in the tracking weight. This is correct except for one step.....the auto mechanism must be eliminated to perform this correctly. There is an online manual that explains how to do that.
My Pioneer currently uses a Grado Gold cartridge. I feed it into a Graham Slee phono stage. I am happy with the sound. Your Pioneer is a good table. Sure, there are many more out there that perform better and with more expensive cartridges. But, it is a good table. Stay with it if you're happy.

@jrpnde - thanks for the notes! That's awesome that you've owned your PL350 since new! I did see some notes about the weak tonearm before picking mine up so I checked and it's all good. The table was serviced by a local shop (Austin Stereo) before I snagged it but probably worth double checking the cart set up. I mean overall the system sounds good, I'm just looking for great, on a budget. 

Have you tried other phono stages? How would you describe the sound of the Grahm Slee?  

+2 on the Nagaoka MP110. Great cartridge and value, very smooth and balanced. I have that one plus the MP150 and MP200. All are superb.

Thanks @stevecham the Nagaoka will be high on my list. At that price point it and with all the positive reviews I've read it seems like a nobrainer. 

@designtaylor 
Also very important... religiously clean your stylus per your cartridge manufacturers recommendation. Long story short, I ran out of mine and started using distilled water, which was not up to the task. I tried many time consuming and expensive things to get my sound quality back, all because of a little bottle of fluid. Sigh...