Vintage Dual 1228 Turntable : worth to upgrade cartridge ?

I have from my college days (1977) Dual 1228 with original fitted Shure M95ED cartridge .  It is maintained well , not used for about 20 years in between , but just cleaned , lubricated , resoldered some loose connections , etc and just runs great for me. 
I am not too much into analogue , so not planning to do big time investment into new TT , Phono preamp , but if  can get larger benefit by replacing the Shure M 95 ED with some new cart upto $ 200-300 , I would like to do it . 
My Phono preamp is very low end Cambridge Audio MM 551P . To go with new upgraded cartridge , I may even consider to buy used Phono with budget of $1000 . 
I would appreciate advice form FMs here , where I should spend my money to get most benifit on sound improvement . New cart , new phono ? 

Sorry for the late reply, I wasn't able to complete the process and set up my Dual 1228 correctly until just recently. Thank you so much for all your excellent advice Radni and Cliffkhz. I opted to send it to Bill at, where it only needed to be cleaned and lubed, just as you thought. Bill was great, also checked my cartridges and found that one stylus was bad and suggested a nice replacement. He then guided me patiently and kindly in setting it up properly when I hit a roadblock. I have now been listening to my old vinyl for 2 weeks and can't get enough! The sheer musicality on good recordings is amazing, so much so that the rare pops that bothered me eons ago and hastened my switch to digital are barely even noticed now. The very slightly smaller dynamic range of analog and more frequent intermissions' needed to flip the LPs on symphonic works or operas do make listening slightly disjointed, but for most rock, blues, folk and jazz are not issues at all. It is really fantastic to have the wonderful analog sound of vinyl back, all thanks to you and Bill!
I'm glad to hear your happy, Bill sure is knowledgable with Dual.Happy listening!
rwmeditz, I feel happy that your efforts to restore your Dual TT paid off well. My experience has also been similar.
I had replaced the old stylus with a JICO ( Japan) stylus model
N95HE , cost me $96 plus shipping cost $15 , and got it delivered to me in less than 5 days. It sounds amazing! I was , initially, going to change the cartridge, but got some useful advice from this forum to first only change the stylus. And it has worked so well for me. I would suggest to consider to go for such stylus repalcement in future whenever it is right time for you.
BTW, website of JICO is worth visiting :
Enjoy your Dual TT!
Thanks Radni, that's exactly what I did, replaced the stylus with a JICO. Looked into NOS, shocked at the prices, so went with JICO. So grateful for all the helpful advice from the community here.