@ Digital X:
First: Welcome to the forum!
Next: Your small story about investigating laptops and then finally getting a ThinkPad reminds me of a saying that we sometime see here... about folks who get a ThinkPad:
Once you go black, you never go back! 
To reply to your question; how easy upgrades are? Very easy! This is not the least because IBM has established a tradition for making it possible (and, fortunately also: Easy!) for custumers to upgrade and service their own laptops, not the least through putting at the end-users disposal outstanding documentation... such as first and foremost the
Hardware Maintenance Manual - ThinkPad T60, T60p 14.1" and 15" (with 4:3 format LCD's), secondly their online look-up
System service parts - ThinkPad T60, T60p (standard screen), their movies (!!) at
How to remove and install hardware - ThinkPad T60, T60p, their
Lenovo T60/T60p Service Training Videos plus
T60 @ Wiki + Youtube + (not the least!) this forum!
About upgrading RAM, a T60 can hold 4 GB (2 x 2 GB), but owing to hardware limitations, only 3 GB can be used, so there's really no significant reason to get more than 1 GB + 2 GB (the T60 has two slots for RAM). For my recommendations with respect to RAM, see the thread
T60 upgrade question.
As explained above you can also upgrade the CPU (but check your motherboard-revision first!); see e.g the thread
T60 CPU upgrade question. In my opinion there is however relatively little achieved, in terms of "daily-day improvements" in speed, by upgrading from e.g. a T2400 Core Duo to a T7200 Core 2 Duo. You will certainly experience a
significant larger boost in speed and overall performance by upgrading your current (mechanical rotating and therefore relatively slow) HDD to a SSD... see e.g. the above-mentioned thread. Go grab a cheap e.g 80 GB or 120 GB SSD on eBay, or, much more recommended: Go to
The Marketplace on this (fantastic!) forum!
You can use internal and external HDD's (and external USB-keys) up to 2 TB++ in size if you run NTFS (the latter which I would assume you do). If getting a new external USB-based device, keep in mind that USB 3.0 are significantly faster compared to USB 2.0 (so a USB 3.0 device will be more pleasant to use years ahead, with e.g. newer HW which will fully support USB 3.0). If getting a new HDD, it will need to be a 2.5" SATA drive (both 7 mm and 9.5 mm drives will work). Since T60's are "only" capable of supporting SATA-I speeds you won't achieve the full benefit of e.g. the latest-generation (and hence fastest!) SATA-III drives if putting it in a T60, but since e.g. SATA-III drives are backward compatible, they can be used just fine in a SATA-I T60 (and, again, you can re-use a fast SATA-III drive in a future machine).
About the Ultrabay-option mentioned above, see
SSD Upgrade for T60p How-to? Need suggestions pointing to
this post. You can get an Ultrabay HDD (Remenber: Get a SATA, not a PATA!!) caddy for T60's for around $10 on eBay.
Be aware that Microsoft will terminate their "Extended Life Support" on Windows XP (SP3) on April 8, 2014 (see
Windows lifecycle fact sheet). This meas that if you go continue with XP, then you will be "unsafe" after that date... since security issues etc. are no longer being cured, on XP. Because of this I guess that many users are considering ugrading fro mXp to e.g. Windows 7 or 8 - depending on their need, wallet, and whether their more or less old hardware will actually support Win 7 or 8
Again: Congratulations with your T60, and good luck with it; I am sure you will enjoy it - especially if getting a SSD!
Johan